Network Handover Method and Network Device

ABSTRACT

This application discloses a network handover method and a network device, to resolve a problem that a terminal device cannot perform an IMS voice service. The method includes: receiving, by a first access network device, first session management information from a session management network element, where the first session management information is for requesting to set up a user plane tunnel for a voice service of a terminal device; and sending, by the first access network device, a handover request to a first mobility management network element based on the first session management information and a first condition, where the handover request is for handing over the terminal device from a first network to a second network, and the first condition includes that the voice service cannot be performed in the first network. This application is applicable to the field of communication technologies.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/CN2018/091759, filed on Jun. 19, 2018, which claims priority toChinese Patent Application No. 201710577345.7, filed on Jul. 14, 2017,and Chinese Patent Application No. 201710693995.8, filed on Aug. 14,2017. Chinese Patent Application No. 201710693995.8 claims priority toChinese Patent Application No. 201710577345.7. All of the aforementionedpatent applications are hereby incorporated by reference in theirentireties.

TECHNICAL FIELD

This application relates to the field of communication technologies, andin particular, to a network handover method and a network device.

BACKGROUND

Currently, on one hand, many operators have not intended or do notintend to deploy an Internet Protocol Multimedia Subsystem (IMS) voiceservice in a fifth-generation (5G) wireless communications system. Onthe other hand, most frequency spectrums in a 5G network are on highfrequency bands, a high frequency signal has small coverage, while theIMS voice service requires relatively large signal coverage.Consequently, a terminal device in the 5G network cannot perform the IMSvoice service.

SUMMARY

Embodiments of this application provide a network handover method andapparatus, to resolve a problem that a terminal device cannot perform anIMS voice service.

To achieve the foregoing objective, the embodiments of this applicationprovide the following technical solutions.

According to a first aspect, a network handover method is provided. Themethod includes: receiving, by a first access network device, firstsession management information from a session management networkelement, where the first session management information is forrequesting to set up a user plane tunnel for a voice service of aterminal device; and sending, by the first access network device, ahandover request to a first mobility management network element based onthe first session management information and a first condition, wherethe handover request is for handing over the terminal device from afirst network to a second network, and the first condition includes thatthe voice service cannot be performed in the first network.

In the method according to the first aspect, when the terminal device inthe first network performs a voice service, the terminal device may behanded over to the second network, so that the terminal device mayperform the voice service in the second network, to resolve a problemthat the terminal device in the first network cannot perform the voiceservice.

In a possible design, the first condition further includes at least oneof that the second network supports the voice service, that signalquality of the second network satisfies a requirement of the voiceservice, and a network device of the first network and the terminaldevice supports handover between the first network and the secondnetwork.

In a possible design, the method further includes: sending, by the firstaccess network device, second session management information to thesession management network element, where the second session managementinformation includes information indicating that setup of the user planetunnel of the voice service is accepted. In such possible design, anetwork element in the first network sets up the user plane tunnel forthe voice service of the terminal device.

In a possible design, before the sending, by the first access networkdevice, second session management information to the session managementnetwork element, the method further includes: determining, by the firstaccess network device based on the first session management informationand the first condition, not to allocate a radio resource to the userplane tunnel of the voice service. In such possible design, a networkelement in the first network sets up the user plane tunnel for the voiceservice of the terminal device.

In a possible design, the method further includes: sending, by the firstaccess network device, third session management information to thesession management network element, where the third session managementinformation includes information indicating that setup of the user planetunnel of the voice service is rejected. In such possible design, afterthe terminal device completes handover from the first network to thesecond network, a network element in the second network sets up the userplane tunnel for the voice service of the terminal device.

In a possible design, the information indicating that setup of the userplane tunnel of the voice service is rejected includes reasoninformation indicating rejection. In such possible design, after theterminal device completes handover from the first network to the secondnetwork, a network element in the second network sets up the user planetunnel for the voice service of the terminal device.

In a possible design, the reason information is that the terminal deviceis performing a handover process. In such possible design, after theterminal device completes handover from the first network to the secondnetwork, a network element in the second network sets up the user planetunnel for the voice service of the terminal device.

In a possible design, the first condition further includes that thevoice service cannot be performed in the second network, and the methodfurther includes: sending, by the first access network device, fourthsession management information to the session management networkelement, where the fourth session management information includesinformation indicating that setup of the user plane tunnel of the voiceservice is rejected, and the information indicating that setup of theuser plane tunnel of the voice service is rejected includes that a radioresource is insufficient. In such possible design, after the terminaldevice completes handover from the first network to the second network,a network element in the second network does not set up the user planetunnel for the voice service of the terminal device.

According to a second aspect, a network handover method is provided. Themethod includes: receiving, by a session management network element,third session management information sent from a first access networkdevice, where the third session management information includes reasoninformation indicating that setup of a user plane tunnel of a voiceservice of a terminal device is rejected; determining, by the sessionmanagement network element, that the terminal device has handed overfrom a first network to a second network; and setting up, by the sessionmanagement network element, the user plane tunnel for the voice servicein the second network based on the reason information.

In the method according to the second aspect, when the terminal devicein the first network is handed over from the first network to the secondnetwork, the session management network element sets up the user planetunnel for the voice service in the second network based on the reasoninformation, and if the voice service may be performed in the secondnetwork, the user plane tunnel may be set up successfully, so that theterminal device may perform the voice service in the second network, toresolve a problem that the terminal device in the first network cannotperform the voice service.

In a possible design, the reason information is that the terminal deviceis performing a handover process.

In a possible design, the setting up, by the session management networkelement, the user plane tunnel for the voice service in the secondnetwork based on the reason information includes: setting up, by thesession management network element, the user plane tunnel for the voiceservice in the second network after a first preset period based on thereason information and a second condition, where the second conditionincludes that the voice service cannot be performed in the secondnetwork. In such possible design, the first preset period is reservedfor the terminal device to complete joint registration, and by properlysetting the first preset period, after completing the jointregistration, the terminal device may initiate a CSFB call, therebyensuring that the terminal device successfully initiates the CSFB call.

In a possible design, the setting up, by the session management networkelement, the user plane tunnel for the voice service in the secondnetwork after a first preset period based on the reason information anda second condition includes: sending, by the session management networkelement, a tunnel setup request to a serving gateway of the secondnetwork after the first preset period based on the reason informationand the second condition, where the tunnel setup request is forrequesting to set up the user plane tunnel of the voice service.

In a possible design, the setting up, by the session management networkelement, the user plane tunnel for the voice service in the secondnetwork after a first preset period based on the reason information anda second condition includes: sending, by the session management networkelement, a tunnel setup request to a serving gateway of the secondnetwork based on the reason information, where the tunnel setup requestis for requesting to set up the user plane tunnel of the voice service;receiving, by the session management network element, a tunnel setupresponse sent from the serving gateway, where the tunnel setup responseincludes information indicating that setup of the user plane tunnel ofthe voice service is rejected; and sending, by the session managementnetwork element, a first message to a policy function control networkelement after the first preset period based on the second condition,where the first message is for indicating that setup of the user planetunnel of the voice service fails.

According to a third aspect, a network handover method is provided. Themethod includes: receiving, by a second mobility management networkelement, a tunnel setup request sent by a serving gateway, where thetunnel setup request is for requesting to set up a user plane tunnel ofa voice service for a terminal device; and sending, by the secondmobility management network element, a tunnel setup response to theserving gateway based on a second condition, where the tunnel setupresponse includes information indicating that setup of the user planetunnel of the voice service of the terminal device is rejected, and thesecond condition includes that the voice service cannot be performed ina second network.

In the method according to the third aspect, when the voice servicecannot be performed in the second network, the second mobilitymanagement network element sends the tunnel setup response to theserving gateway, so that the serving gateway transfers, to an IMSnetwork, the information indicating that setup of the user plane tunnelis rejected, so that the IMS network may trigger a CSFB service of theterminal device, and then the terminal device performs a CS voiceservice.

In a possible design, the second condition further includes that theterminal device is a terminal device handed over from a first network tothe second network.

In a possible design, before the sending, by the second mobilitymanagement network element, a tunnel setup response to the servinggateway based on a second condition, the method further includes:determining, by the second mobility management network element, that theterminal device completes joint registration. Such possible design mayensure that the terminal device successfully initiates a CSFB call.

In a possible design, the determining, by the second mobility managementnetwork element, that the terminal device completes joint registrationincludes: determining, by the second mobility management networkelement, that a joint registration complete message from the terminaldevice is received.

According to a fourth aspect, a network handover method is provided. Themethod includes: sending, by a second mobility management networkelement, a radio tunnel setup request to a second access network device,where the radio tunnel setup request is for requesting to set up a userplane tunnel for a voice service of a terminal device; receiving, by thesecond mobility management network element, a radio tunnel setupresponse sent by the second access network device, where the radiotunnel setup response includes information indicating that setup of theuser plane tunnel of the voice service of the terminal device isrejected; and sending, by the second mobility management networkelement, a tunnel setup response to the serving gateway based on asecond condition, where the tunnel setup response includes theinformation indicating that setup of the user plane tunnel of the voiceservice of the terminal device is rejected, and the second conditionincludes that the voice service cannot be performed in a second network.

In the method according to the fourth aspect, the second mobilitymanagement network element sends, to the serving gateway, theinformation indicating that setup of the user plane tunnel is rejected,so that the serving gateway transfers, to an IMS network, theinformation indicating that setup of the user plane tunnel is rejected,the IMS network may trigger a CSFB service of the terminal device, andthen the terminal device performs a CS voice service.

In a possible design, the second condition further includes that theterminal device is a terminal device handed over from a first network tothe second network.

In a possible design, the sending, by the second mobility managementnetwork element, a tunnel setup response to the serving gateway based ona second condition includes: determining, by the second mobilitymanagement network element, that the second condition is satisfied andthe terminal device completes joint registration; and sending, by thesecond mobility management network element, the tunnel setup response tothe serving gateway. Such possible design may ensure that the terminaldevice successfully initiates a CSFB call.

In a possible design, the determining, by the second mobility managementnetwork element, that the terminal device completes joint registrationincludes: determining, by the second mobility management networkelement, that a registration complete message from the terminal deviceis received.

According to a fifth aspect, a network handover method is provided. Themethod includes: receiving, by a second mobility management networkelement, a tunnel setup request sent by a serving gateway, where thetunnel setup request is for requesting to set up a user plane tunnel ofa voice service for user equipment, namely, a terminal device; if asecond condition is satisfied, determining, by the second mobilitymanagement network element, that the terminal device completes jointregistration, where the second condition includes that the voice servicecannot be performed in a second network; sending, by the second mobilitymanagement network element, a radio tunnel setup request to a secondaccess network device, where the radio tunnel setup request is forrequesting to set up the user plane tunnel for the voice service of theterminal device; receiving, by the second mobility management networkelement, a radio tunnel setup response sent by the second access networkdevice, where the radio tunnel setup response includes informationindicating that setup of the user plane tunnel of the voice service ofthe terminal device is rejected; and sending, by the second mobilitymanagement network element, a tunnel setup response to the servinggateway based on the radio tunnel setup response, where the tunnel setupresponse includes the information indicating that setup of the userplane tunnel of the voice service of the terminal device is rejected.

In the method according to the fifth aspect, after determining that theterminal device completes joint registration, the second mobilitymanagement network element sends, to the serving gateway, theinformation indicating that setup of the user plane tunnel is rejected,so that the serving gateway transfers, to an IMS network, theinformation indicating that setup of the user plane tunnel is rejected,the IMS network may trigger a CSFB service of the terminal device, andthen the terminal device performs a CS voice service, to further ensurethat the terminal device successfully initiates a CSFB call.

In a possible design, the second condition further includes that theterminal device is a terminal device handed over from a first network tothe second network.

In a possible design, the determining, by the second mobility managementnetwork element, that the terminal device completes joint registrationincludes: determining, by the second mobility management networkelement, that a registration complete message sent by the terminaldevice is received.

According to a sixth aspect, a network handover method is provided. Themethod includes: receiving, by a second access network device, a radiotunnel setup request sent by a second mobility management networkelement, where the radio tunnel setup request is for requesting thesecond access network device to set up a user plane tunnel of a voiceservice for a terminal device; and sending, by the second access networkdevice, a radio tunnel setup response to the second mobility managementnetwork element based on a second condition, where the radio tunnelsetup response includes information indicating that setup of the userplane tunnel of the voice service is rejected, and the second conditionincludes that the voice service cannot be performed in a second network.

In the method according to the sixth aspect, when the voice servicecannot be performed in the second network, the second access networkdevice rejects allocating a radio resource to the user plane tunnel,adds, to the radio tunnel setup response, the information indicatingthat setup of the user plane tunnel of the voice service is rejected,and sends the radio tunnel setup response to the second mobilitymanagement network element, and the second mobility management networkelement may transfer, to an IMS network, the information indicating thatsetup of the user plane tunnel is rejected, so that the IMS networktriggers a CSFB service of the terminal device, and then the terminaldevice performs a CS voice service.

In a possible design, the second condition further includes that theterminal device is a terminal device handed over from a first network tothe second network.

According to a seventh aspect, a network handover method is provided.The method includes: receiving, by an IMS network element, a requestmessage for setting up a voice service for a terminal device in a PSdomain; determining, by the IMS network element, that a voice servicecannot be set up in a PS domain of a first network or a second network;determining, by the IMS network element, that the terminal devicecompletes joint registration; and triggering, by the IMS networkelement, a CSFB service of the terminal device.

In the method according to the seventh aspect, when determining that avoice service cannot be set up in the PS domain of the first network orthe second network, and the terminal device completes jointregistration, the IMS network element triggers the CSFB service of theterminal device. Because that the terminal device completes jointregistration is a necessary condition on which the terminal deviceinitiates a CSFB call, the method according to the seventh aspect mayfurther ensure that the terminal device successfully initiates the CSFBcall, so that the terminal device performs a CS voice service.

In a possible design, after the receiving, by an IMS network element, arequest message for setting up a voice service for a terminal device ina PS domain, the method further includes: determining, by the IMSnetwork element that the terminal device is registered with the firstnetwork.

In a possible design, the determining, by the IMS network element, thata voice service cannot be set up in a PS domain of a first network or asecond network includes: obtaining, by the IMS network element, voiceservice setup failure information that is from the PS domain of thefirst network; or obtaining, by the IMS network element, voice servicesetup failure information that is from the PS domain of the secondnetwork.

In a possible design, the determining, by the IMS network element, thatthe terminal device completes joint registration includes: after asecond preset period since the IMS network element determines that thevoice service cannot be set up in the PS domain of the first network orthe second network, determining, by the IMS network element, that theterminal device completes joint registration; or determining, by the IMSnetwork element based on registration location information of theterminal device obtained from a policy control network element or a homesubscriber server, that the terminal device completes jointregistration.

In a possible design, the terminal device is a calling terminal deviceof the voice service, and the triggering, by the IMS network element, aCSFB service of the terminal device includes: sending, by the IMSnetwork element, a second message to the calling terminal device, wherethe second message is for triggering the calling terminal device toinitiate a CSFB calling service.

In a possible design, the terminal device is a called terminal device ofthe voice service, and the triggering, by the IMS network element, aCSFB service of the terminal device includes: sending, by the IMSnetwork element, a third message to a terminating access domainselection device, where the third message is for indicating that setupof a called voice service in the PS domain of the first network or thesecond network fails, so that the terminating access domain selectiondevice triggers a CSFB called voice service of the called terminaldevice.

According to an eighth aspect, a network handover method is provided.The method includes: receiving, by a second mobility management networkelement, a fourth message sent by a first mobility management networkelement, where the fourth message is for determining that a terminaldevice completes relocation; and sending, by the second mobilitymanagement network element, a tunnel modification request to a servinggateway based on a second condition, where the tunnel modificationrequest is for modifying signaling plane tunnel information of a voiceservice of the terminal device, and the second condition includes thatthe voice service cannot be performed in a second network.

In the method according to the eighth aspect, when the voice servicecannot be performed in the second network, the second mobilitymanagement network element sends the tunnel modification request to theserving gateway, the serving gateway forwards the tunnel modificationrequest to a session management network element, and because only afterreceiving the tunnel modification request, the session managementnetwork element sends, to the terminal device, a message (that is, thefollowing second message) for triggering the terminal device to initiatea CSFB call, in the method according to the eighth aspect, the terminaldevice may be triggered to initiate the CSFB call when the voice servicecannot be performed in the second network, so that the terminal deviceperforms a CS voice service.

In a possible design, the sending, by the second mobility managementnetwork element, a tunnel modification request to a serving gatewaybased on a second condition includes: determining, by the secondmobility management network element, that the second condition issatisfied and the UE completes joint registration; and sending, by thesecond mobility management network element, the tunnel modificationrequest to the serving gateway. Such possible design may ensure that theterminal device successfully initiates a CSFB call.

According to a ninth aspect, a network registration method is provided.The method includes: receiving, by a first mobility management networkelement, a registration request from a terminal device, where theregistration request is for requesting to register the terminal devicewith a first network; and sending, by the first mobility managementnetwork element, first indication information to the terminal device ifa third condition is satisfied, where the first indication informationis for indicating that a voice service of a PS domain is supported, andthe third condition includes that the first network supports movement ofthe terminal device to a second network. In the method according to theninth aspect, regardless of whether the first network supports the voiceservice of the terminal device, provided that the first network has acapability of enabling, in a handover or another manner, the terminaldevice to fall back to a network that can provide the voice service whenthe terminal device initiates the voice service, when the terminaldevice is registered with the first network, the first mobilitymanagement network element indicates that the voice service in the PSdomain of the terminal device is supported, thereby ensuring that theterminal device is registered with and camps on the first network anddoes not reselect another network, thereby saving network resources, andensuring that the terminal device normally performs a PS domain voiceservice registration process and initiates a PS domain voice callprocess by using the first network.

In a possible design, the first network and the second network have asame core network and different access types; or the first network andthe second network have different core networks and different accesstypes.

In a possible design, that the first network supports movement of theterminal device to a second network includes that a first access networkdevice in the first network supports, when receiving first sessionmanagement information from a session management network element, accessof the terminal device to the second network, where the first sessionmanagement information is for requesting to set up a user plane tunnelfor the voice service of the terminal device.

In a possible design, the third condition further includes at least oneof that a handover interoperation is supported between the first networkand the second network, and that the second network can support thevoice service of the PS domain.

In a possible design, that a handover interoperation is supportedbetween the first network and the second network includes that ahandover interoperation interface exists between the first network andthe second network.

In a possible design, before the sending, by the first mobilitymanagement network element, first indication information to the terminaldevice, the method further includes: determining, by the first mobilitymanagement network element, that the terminal device is a voice servicecentric terminal device. Because a data service centric terminal deviceis unrelated to the voice service, the optional method may distinguishterminal types. The data service centric terminal device does not needto perform this set of solution, that is, does not need to determinewhether the third condition is satisfied, and does not need to send thefirst indication information either.

In a possible design, the registration request carries indicationinformation indicating that the terminal device is a voice servicecentric terminal device, and the determining, by the first mobilitymanagement network element, that the terminal device is a voice servicecentric terminal device includes: determining, by the first mobilitymanagement network element based on the registration request, that theterminal device is a voice service centric terminal device.

In a possible design, before the sending, by the first mobilitymanagement network element, first indication information to the terminaldevice, the method further includes: determining, by the first mobilitymanagement network element, that the terminal device supports movementfrom the first network to the second network in a voice serviceinitiation process and continuity of the voice service initiationprocess by using the second network. Because the method according to thefirst aspect may be implemented only when the terminal device supportsmovement from the first network to the second network in a voice serviceinitiation process and continuity of the voice service initiationprocess by using the second network, the information may be determinedin advance, to avoid impact on a terminal device that cannot supportmovement from the first network to the second network in a voice serviceinitiation process and continuity of the voice service initiationprocess by using the second network.

In a possible design, the registration request further carriesindication information indicating that the terminal device supportsmovement from the first network to a second network in a voice serviceinitiation process and continuity of the voice service initiationprocess by using the second network, and the determining, by the firstmobility management network element, that the terminal device supportsmovement from the first network to the second network in a voice serviceinitiation process and continuity of the voice service initiationprocess by using the second network includes: determining, by the firstmobility management network element based on the registration request,that the terminal device supports movement from the first network to thesecond network in a voice service initiation process and continuity ofthe voice service initiation process by using the second network.

According to a tenth aspect, a network registration method is provided.The method includes: generating, by a terminal device, a registrationrequest, where the registration request is for requesting to registerthe terminal device with a first network, and the registration requestcarries indication information indicating that the terminal device is avoice service centric terminal device; and sending, by the terminaldevice, the registration request to the first mobility managementnetwork element.

In a possible design, the registration request further includesindication information indicating that the terminal device supportsmovement from the first network to a second network in a voice serviceinitiation process and continuity of the voice service initiationprocess by using the second network.

In a possible design, the first network and the second network have asame core network and different access types; or the first network andthe second network have different core networks and different accesstypes.

According to an eleventh aspect, a network device is provided. Thenetwork device may be a first access network device, a sessionmanagement network element, a second mobility management networkelement, a second access network device, an IMS network element, or afirst mobility management network element. When the network device isthe first access network device, the network device has a function ofimplementing any method according to the first aspect; when the networkdevice is the session management network element, the network device hasa function of implementing any method according to the second aspect;when the network device is the second mobility management networkelement, the network device has a function of implementing any methodaccording to the third aspect, the fourth aspect, the fifth aspect, orthe eighth aspect; when the network device is the second access networkdevice, the network device has a function of implementing any methodaccording to the sixth aspect; when the network device is the IMSnetwork element, the network device has a function of implementing anymethod according to the seventh aspect; and when the network device isthe first mobility management network element, the network device has afunction of implementing any method according to the ninth aspect. Afunction implemented by the network device may be implemented by usinghardware, or may be implemented by executing corresponding software byusing hardware. The hardware or software includes one or more functionalunits corresponding to the foregoing method.

According to a twelfth aspect, a terminal device is provided. Theterminal device has a function of implementing any method according tothe tenth aspect. A function implemented by the terminal device may beimplemented by using hardware, or may be implemented by executingcorresponding software by using hardware. The hardware or softwareincludes one or more functional units corresponding to the methodaccording to the tenth aspect.

According to a thirteenth aspect, a network device is provided. Thenetwork device includes: a memory, a processor, and a communicationsinterface. The memory is configured to store a computer executableinstruction, and the processor executes the computer executableinstruction stored in the memory, and enables, by using thecommunications interface, the network device to implement the methodaccording to the foregoing aspect. Specifically, the network device maybe a first access network device, a session management network element,a second mobility management network element, a second access networkdevice, an IMS network element, or a first mobility management networkelement. When the network device is the first access network device, thenetwork device may implement any method according to the first aspect;when the network device is the session management network element, thenetwork device may implement any method according to the second aspect;when the network device is the second mobility management networkelement, the network device may implement any method according to thethird aspect, the fourth aspect, the fifth aspect, or the eighth aspect;when the network device is the second access network device, the networkdevice may implement any method according to the sixth aspect; when thenetwork device is the IMS network element, the network device mayimplement any method according to the seventh aspect; when the networkdevice is the first mobility management network element, the networkdevice has a function of implementing any method according to the ninthaspect.

According to a fourteenth aspect, a terminal device is provided. Theterminal device includes: a memory, a processor, and a communicationsinterface. The memory is configured to store a computer executableinstruction, and the processor executes the computer executableinstruction stored in the memory, and enables, by using thecommunications interface, the terminal device to implement any methodaccording to the tenth aspect.

According to a fifteenth aspect, a computer readable storage medium isprovided. The computer readable storage medium includes an instruction,and when run on a computer, the instruction enables the computer toperform any method according to the first aspect.

According to a sixteenth aspect, a computer readable storage medium isprovided. The computer readable storage medium includes an instruction,and when run on a computer, the instruction enables the computer toperform any method according to the second aspect.

According to a seventeenth aspect, a computer readable storage medium isprovided. The computer readable storage medium includes an instruction,and when run on a computer, the instruction enables the computer toperform any method according to the third aspect.

According to an eighteenth aspect, a computer readable storage medium isprovided. The computer readable storage medium includes an instruction,and when run on a computer, the instruction enables the computer toperform any method according to the fourth aspect.

According to a nineteenth aspect, a computer readable storage medium isprovided. The computer readable storage medium includes an instruction,and when run on a computer, the instruction enables the computer toperform any method according to the fifth aspect.

According to a twentieth aspect, a computer readable storage medium isprovided. The computer readable storage medium includes an instruction,and when run on a computer, the instruction enables the computer toperform any method according to the sixth aspect.

According to a twenty first aspect, a computer readable storage mediumis provided. The computer readable storage medium includes aninstruction, and when run on a computer, the instruction enables thecomputer to perform any method according to the seventh aspect.

According to a twenty second aspect, a computer readable storage mediumis provided. The computer readable storage medium includes aninstruction, and when run on a computer, the instruction enables thecomputer to perform any method according to the eighth aspect.

According to a twenty third aspect, a computer readable storage mediumis provided. The computer readable storage medium includes aninstruction, and when run on a computer, the instruction enables thecomputer to perform any method according to the ninth aspect.

According to a twenty fourth aspect, a computer readable storage mediumis provided. The computer readable storage medium includes aninstruction, and when run on a computer, the instruction enables thecomputer to perform any method according to the tenth aspect.

According to a twenty fifth aspect, a computer program product includingan instruction is provided. When run on a computer, the instructionenables the computer to perform any method according to the firstaspect.

According to a twenty sixth aspect, a computer program product includingan instruction is provided. When run on a computer, the instructionenables the computer to perform any method according to the secondaspect.

According to a twenty seventh aspect, a computer program productincluding an instruction is provided. When run on a computer, theinstruction enables the computer to perform any method according to thethird aspect.

According to a twenty eighth aspect, a computer program productincluding an instruction is provided. When run on a computer, theinstruction enables the computer to perform any method according to thefourth aspect.

According to a twenty ninth aspect, a computer program product includingan instruction is provided. When run on a computer, the instructionenables the computer to perform any method according to the fifthaspect.

According to a thirtieth aspect, a computer program product including aninstruction is provided. When run on a computer, the instruction enablesthe computer to perform any method according to the sixth aspect.

According to a thirty first aspect, a computer program product includingan instruction is provided. When run on a computer, the instructionenables the computer to perform any method according to the seventhaspect.

According to a thirty second aspect, a computer program productincluding an instruction is provided. When run on a computer, theinstruction enables the computer to perform any method according to theeighth aspect.

According to a thirty third aspect, a computer program product includingan instruction is provided. When run on a computer, the instructionenables the computer to perform any method according to the ninthaspect.

According to a thirty fourth aspect, a computer program productincluding an instruction is provided. When run on a computer, theinstruction enables the computer to perform any method according to thetenth aspect.

For a technical effect brought by any design manner in the eleventhaspect to the thirty fourth aspect, refer to a technical effect broughtby different design manners in the first aspect to the tenth aspect.Details are not described herein again.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic architectural diagram of a 5G network according toan embodiment of this application;

FIG. 1A is a schematic diagram of access of a terminal device to a 5G CNaccording to an embodiment of this application;

FIG. 2 is a schematic architectural diagram of an EPS network accordingto an embodiment of this application;

FIG. 3 is a schematic diagram of a network architecture obtained afterintegrating a 5G network and an EPS network according to an embodimentof this application;

FIG. 4 is a schematic diagram of communication between an EPS networkand an IMS network according to an embodiment of this application;

FIG. 5 is a schematic diagram of performing IMS voice communicationbetween terminal devices according to an embodiment of this application;

FIG. 6 is a schematic composition diagram of a network device accordingto an embodiment of this application;

FIG. 6A is a schematic composition diagram of a terminal deviceaccording to an embodiment of this application;

FIG. 7 is an interaction flowchart of a network handover methodaccording to an embodiment of this application;

Each of FIG. 8 and FIG. 9 is a flowchart of a network handover processaccording to an embodiment of this application;

Each of FIG. 10 to FIG. 12 is a schematic diagram of a user plane tunnelsetup process according to an embodiment of this application;

Each of FIG. 13 to FIG. 17 is a schematic diagram of a method forensuring that a terminal device successfully initiates a CSFB callaccording to an embodiment of this application;

Each of FIG. 18A and FIG. 18B to FIG. 28 is an interaction flowchart ofa network handover method according to an embodiment of thisapplication;

FIG. 28A is a flowchart of a network registration method according to anembodiment of this application;

FIG. 28B is a flowchart of another network registration method accordingto an embodiment of this application;

FIG. 29 is a schematic diagram of a network device according to anembodiment of this application; and

FIG. 29A is a schematic diagram of a terminal device according to anembodiment of this application.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The following describes the technical solutions in the embodiments ofthis application with reference to the accompanying drawings in theembodiments of this application. In description of this application, “/”means “or” unless otherwise specified. For example, A/B may represent Aor B. In this specification, “and/or” describes only an associationrelationship for describing associated objects and represents that threerelationships may exist. For example, A and/or B may represent thefollowing three cases: Only A exists, both A and B exist, and only Bexists. Additionally, in the description of this application, “aplurality of” means two or more than two.

A method provided in the embodiments of this application may be appliedto various communications systems, such as: a Global System for MobileCommunications (GSM), a Code Division Multiple Access (CDMA) system, aWideband Code Division Multiple Access (WCDMA), a general packet radioservice (GPRS), an LTE system, and a Universal Mobile TelecommunicationSystem (UMTS), an EPS network, a 5G network, and a future wirelesscommunications system.

Specifically, the method provided in the embodiments of this applicationmay be applied to a 5G network shown in FIG. 1. As shown in FIG. 1, the5G network may include: an authentication server function (AUSF) networkelement, an access and mobility management function (AMF) networkelement, a data network (DN), a unified data management (UDM) networkelement, a policy control function (PCF) network element, a (radio)access network ((R)AN) device, a user plane function (UPF) networkelement, a terminal device, an application function AF) network element,and a session management function (SMF) network element.

The (R)AN device of the 5G network may be a next generation (NG) RANdevice, or may be an evolved universal terrestrial radio access network(E-UTRAN) device, and the 5G network may be connected to both of theforegoing two access network devices, that is, the 5G network may bothhave two different radio access types (RAT). Referring to FIG. 1A, aterminal device may access a 5G core network (CN) by using an E-UTRANdevice or an NG RAN device. In this embodiment of this application, anexample of the method provided in the embodiments of this application isdescribed by using an example in which the terminal device is registeredwith a 5G network through an NG RAN and initiates a voice service in anNG RAN access type.

Control plane (CP) function network elements include: a UDM networkelement, an AUSF network element, a PCF network element, an AMF networkelement, and an SMF network element.

Specifically, main functions of the (R)AN device include: providing awireless connection. Main functions of the UPF network element include:routing and forwarding a data packet, a mobility anchor, an uplinkclassifier to support routing a service flow to a DN, a BP to support amulti-homing packet data unit (PDU) session, and the like. The DN may bean operator service DN, an Internet access service DN, or a third-partyservice DN. Main functions of the AMF network element include: userregistration management, accessibility detection, SMF node selection,mobile state conversion management, and the like. Main functions of theSMF network element include: control setup, modification, and deletionof a session, user plane node selection, and the like. Main functions ofthe PCF network element include: a policy decision point, and providingrules such as a rule based on a service data stream and applicationdetection, a gating control rule, a quality of service (QoS) rule, and aflow-based charging control rule. Main functions of the AF networkelement include: interacting with a 3rd Generation Partnership Project(3GPP) core network to provide a service, to affect service flowrouting, access network capability open-up, policy control, and thelike. Main functions of the AUSF network element include: providing anauthentication service. Main functions of the UDM network elementinclude: storing user subscription data.

The terminal device may also be referred to as user equipment (UE), anaccess terminal, a subscriber unit, a subscriber station, a mobilestation, a mobile console, a remote station, a remote terminal, a mobiledevice, a user terminal, a terminal, a wireless communications device, auser agent, a user apparatus, or the like. The terminal device may be astation (ST) in a wireless local area network (WLAN), a cellular phone,a cordless phone, a Session Initiation Protocol (SIP) phone, a wirelesslocal loop (WLL) station, a personal digital assistant PDA), a handhelddevice having a wireless communication function, a computing device,another processing device connected to a wireless modem, an in-vehicledevice, or a wearable device (which may also be referred to as awearable intelligent device). The terminal device may further be aterminal device in a next generation communications system, for example,a terminal device in 5G or a terminal device in a future evolved publicland mobile network (PLMN), or a terminal device in a new radio (NR)communications system.

Specifically, the method provided in the embodiments of this applicationmay be further applied to an evolved packet system (EPS) network (thatis, a 4G network usually mentioned) shown in FIG. 2. As shown in FIG. 2,the EPS network may include the following plurality of function networkelements: a terminal device, an E-UTRAN (which may be specifically aneNodeB), a serving gateway (SGW), a packet data network gateway (PGW), amobility management entity (MME), a home subscriber server (HSS), amobile switching center (MSC), and a policy and charging rules function(PCRF) network element.

When both a 5G network and an EPS network are deployed, function networkelements in the 5G network and function network elements in the EPSnetwork may be integrated, so that the 5G network may interact with theEPS network. Referring to FIG. 3, function network elements in anintegrated network include: an HSS+UDM function network element (whichis a function network element having both a function of an HSS and afunction of a UDM), a PCF+PCRF function network element (which is afunction network element having both a function of a PCF and a functionof a PCRF), an SMF+PGW-C function network element (which is a functionnetwork element having both a function of an SMF and a function of apacket data network gateway-control plane (PGW-C)), a UPF+PGW-U functionnetwork element (which is a function network element having both afunction of a UPF and a function of a packet data network gateway-userplane (PGW-U)), an SGW, an MME, an E-UTRAN, an AMF, an NG RAN, and aterminal device. In the embodiments in this application, the HSS+UDMfunction network element, the PCF+PCRF function network element, theSMF+PGW-C function network element, and the UPF+PGW-U are described onlyby using an example, and each network element may alternatively be twonetwork elements separated from each other. This is not limited herein.

Currently, relatively mature voice service solutions include an IMSvoice service solution in an EPS network and a circuit switching (CS)voice service solution in a 2G/3G network. Because a 5G network does notsupport IMS and CS voice services in the prior art, an embodiment ofthis application provides a network handover method, so that whenneeding to perform a voice service, a terminal device may be handed overfrom the 5G network to an EPS network or a 2G/3G network to perform thevoice service.

Referring to FIG. 2, if an IP service of an operator is an IMS voiceservice, for a schematic diagram of communication between an EPS networkand an IMS network, refer to FIG. 4. The IMS network includes: aproxy-call session control function (P-CSCF)/session border control(SBC), an interrogating-call session control function(I-CSCF)/serving-call session control function (S-CSCF), a telephonyapplication server (TAS), and an IMS media gateway (IMS MGW). Aterminating access domain selection function module (T-ADS) may beincluded in the TAS. Therefore, a terminating access domain selectiondevice in the following may be considered as the TAS. The TAS mayalternatively be a service centralization and continuity applicationserver (SCC AS).

When terminal devices are performing an IMS voice service, for a processof communication between a calling terminal device and a called terminaldevice, refer to FIG. 5. The process specifically includes: The callingterminal device sends call related information to a calling accessnetwork, the call related information passes through the calling accessnetwork, a calling core network, a calling IMS network, a called IMSnetwork, a called core network, and a called access network and thenreaches the called terminal device, and information fed back by thecalled terminal device to the calling terminal device is sent in anopposite direction.

As shown in FIG. 6, FIG. 6 is a schematic diagram of a hardwarestructure of a network device 60 according to an embodiment of thisapplication. The network device 60 includes at least one processor 601,a communications bus 602, a memory 603, and at least one communicationsinterface 604. The network device 60 may be a first access networkdevice, a second access network device, a first mobility managementnetwork element, a second mobility management network element, a sessionmanagement network element, an IMS network element, a user planegateway, or a serving gateway in this application.

The processor 601 may be a general-purpose central processing unit(CPU), a microprocessor, an application-specific integrated circuit(ASIC), or one or more integrated circuits configured to control programexecution of the solution of this application.

The communications bus 602 may include a path for transmittinginformation between the foregoing components.

By using any apparatus such as a transceiver, the communicationsinterface 604 is configured to communicate with another device orcommunications network such as the Ethernet, a radio access network(RAN), or a wireless local area network (WLAN).

The memory 603 may be a read-only memory (ROM) or another type of staticstorage device that can store static information and a staticinstruction, or a random access memory (RAM) or another type of dynamicstorage device that can store information and an instruction, or may bean electrically erasable programmable read-only memory (EEPROM), acompact disc read-only memory (CD-ROM) or another compact disc storagemedium, optical disc storage medium (including a compact disc, a laserdisc, an optical disc, a digital versatile disc, a Blu-ray disc, and thelike) and magnetic disk storage medium, another magnetic storage device,or any other medium that can be configured to carry or store desiredprogram code in a form of an instruction or a data structure and that isaccessible to a computer, but is not limited thereto. The memory mayindependently exist and be connected to the processor through the bus.Alternatively, the memory may be integrated with the processor.

The memory 603 is configured to store application program code forperforming the solution of this application, and the processor 601controls the performing. The processor 601 is configured to execute theapplication program code stored in the memory 603, thereby implementinga method described below.

During specific implementation, in an embodiment, the processor 601 mayinclude one or more CPUs such as a CPU0 and a CPU1 in FIG. 6.

During specific implementation, in an embodiment, the network device 60may include a plurality of processors such as the processor 601 and aprocessor 608 in FIG. 6. Each of these processors may be a single-core(single-CPU) processor, or may be a multi-core (multi-CPU) processor.The processor herein may be one or more devices, circuits, and/orprocessing cores configured to process data (for example, a computerprogram instruction).

During specific implementation, in an embodiment, the network device 60may further include an output device 605 and an input device 606. Theoutput device 605 communicates with the processor 601, and may displayinformation in various manners.

As shown in FIG. 6A, FIG. 6A is a schematic diagram of a hardwarestructure of a terminal device 60A according to an embodiment of thisapplication. The terminal device 60A includes at least one processor601A, a communications bus 602A, a memory 603A, and at least onecommunications interface 604A.

During specific implementation, in an embodiment, the processor 601A mayinclude one or more CPUs such as a CPU0 and a CPU1 in FIG. 6A.

During specific implementation, in an embodiment, the terminal device60A may include a plurality of processors such as the processor 601A anda processor 608A in FIG. 6A.

During specific implementation, in an embodiment, the terminal device60A may further include an output device 605A and an input device 606A.For an example of a function of each device shown in FIG. 6A and otherdescription, refer to the foregoing description.

In this embodiment of this application, a first network and a secondnetwork may be two networks whose access types are different and whosecore networks are also different, for example, a 5G network having an NGRAN access type and an EPS network having an E-UTRAN access type; or maybe two networks whose access types are different but whose core networksare the same, for example, a 5G network having an NG RAN access type anda 5G network having an E-UTRAN access type. In this embodiment of thisapplication, an example of the method provided in the embodiments ofthis application is described by using that the first network and thesecond network are two networks whose access types are different andwhose core networks are different, but this should not be considered asa limitation on the protection scope of this application. The methodprovided in the embodiments of this application is likewise applicableto two networks whose core networks are the same but whose access typesare different.

The solution provided in the embodiments of this application may bebased on a service-based architecture, and communication between networkelements may be based on a service-based interface or a conventionalnon-service-based interface. This is not specifically limited in theembodiments of this application.

An embodiment of this application provides a network handover method. Asshown in FIG. 7, the method may include the following steps.

701. A first access network device receives first session management(session management, SM for short) information from a session managementnetwork element, where the first session management information is forrequesting to set up a user plane tunnel for a voice service of aterminal device.

Specifically, the first session management information is for requestingto set up the user plane tunnel for the voice service of the terminaldevice in a first network. The voice service of the terminal device maybe an IMS voice service.

The terminal device may be a calling terminal device of the voiceservice, or may be a called terminal device of the voice service. Theuser plane tunnel may be referred to as a QoS flow in a 5G network.

702. The first access network device sends a handover request to a firstmobility management network element based on the first sessionmanagement information and a first condition, where the handover requestis for handing over the terminal device from a first network to a secondnetwork, and the first condition includes that the voice service cannotbe performed in the first network.

The first access network device may be an NG RAN, the session managementnetwork element may be an SMF+PGW-C function network element, the firstmobility management network element may be an AMF, the first network maybe a 5G network, and the second network may be an EPS network.Additionally, the first network and the second network may each be a 5Gnetwork, and have a difference in that the first network and the secondnetwork have different access types.

The first session management information is session managementinformation related to the user plane tunnel of the IMS voice service ofthe terminal device, the first session management information mayinclude QoS parameters of the user plane tunnel of the IMS voice serviceof the terminal device, for example, parameters such as a 5G QoSindicator (referred to as 5QI below) equal to 1, an allocation/retentionpriority (ARP), a guaranteed bit rate (GBR), and a maximum bit rate(MBR), and the first session management information may further includeother information.

The SMF+PGW-C function network element may add the first sessionmanagement information to a session management request with a PDUmodification command and send the session management request to the AMF,and then the AMF adds the first session management information to an N2session request and sends the N2 session request to the NG RAN.

In the method provided in this embodiment of this application, when theterminal device in the 5G network performs a voice service, the terminaldevice may be handed over to the EPS network, so that the terminaldevice may perform the IMS voice service in the EPS network, to resolvea problem that the terminal device in the 5G network cannot perform theIMS voice service.

Based on a rule in a current 5G network protocol, for a voice centricterminal device, if a network side of a 5G network does not support anIMS voice service, the voice centric terminal device should notinitially select NR to access a 5G core network (5GC), or even ifaccessing a 5GC, the voice centric terminal device needs to performreselection to access an E-UTRAN and an evolved packet core (EPC).Therefore, if the 5G network does not support the IMS voice service, theterminal device cannot camp on the 5G network to enjoy a high-speed dataservice. However, in the method provided in this embodiment of thisapplication, when the IMS voice service cannot be performed in the 5Gnetwork, the NG RAN may send a handover request to the AMF, so that theterminal device in the 5G network is handed over from the 5G network tothe EPS network, and the terminal device may perform the IMS voiceservice in the EPS network. Therefore, the terminal device may firstcamp on the 5G network to enjoy a high-speed data service, and then behanded over, when needing to perform the IMS voice service, to the EPSnetwork to perform the IMS voice service.

Further, to ensure that the terminal device in the 5G network maysmoothly perform the voice service, the embodiments of this applicationprovide methods for performing network handover of the terminal devicein two scenarios.

Scenario 1. The voice service cannot be performed in the first network,and the voice service may be performed in the second network.

In the scenario, the first condition may further include at least one ofthat the second network supports the voice service, that signal qualityof the second network satisfies a requirement of the voice service, andthat a network device of the first network and the terminal devicesupports handover between the first network and the second network.

In the scenario, the terminal device may perform network handover ineither of the following manner 1 and manner 2.

Manner 1. The session management network element is notified ofinformation indicating that setup of the user plane tunnel of the voiceservice is accepted, so that the network device in the first networksets up the user plane tunnel for the voice service of the terminaldevice. In a process of handing over the terminal device from the firstnetwork to the second network, the user plane tunnel that is set up bythe network device in the first network for the voice service of theterminal device is switched to a user plane tunnel of a voice service inthe second network.

Referring to FIG. 8, the manner 1 may specifically include the followingsteps.

S11. The first access network device sends second session managementinformation to the session management network element, where the secondsession management information includes the information indicating thatsetup of the user plane tunnel of the voice service is accepted.

Specifically, the first access network device may add the second sessionmanagement information to an N2 session response and send the N2 sessionresponse to the first mobility management network element, and then thefirst mobility management network element adds the second sessionmanagement information to a session management response with a PDUsession modification command and sends the session management responseto the session management network element.

It should be noted that, because the voice service cannot be performedin the first network, although the second session management informationincludes the information indicating that setup of the user plane tunnelof the voice service is accepted, actually the first access networkdevice does not allocate a radio resource to the voice service.

Optionally, referring to FIG. 8, before step S11, the method may furtherinclude: determining, by the first access network device based on thefirst session management information and the first condition, not toallocate a radio resource to the user plane tunnel of the voice service.

S12. The session management network element receives the second sessionmanagement information sent from the first access network device, anddetermines, based on the second session management information,information related to the user plane tunnel of the voice service.

Specifically, the information related to the user plane tunnel of thevoice service of the terminal device may be specifically packetdetection, an enforcement rule, a reporting rule, core network tunnelinformation, and the like related to the user plane tunnel of the voiceservice.

S13. The session management network element sends the informationrelated to the user plane tunnel of the voice service to a user planegateway.

The user plane gateway may be a UPF+PGW-U function network element.

The SMF+PGW-C function network element may add the information relatedto the user plane tunnel of the voice service to an N4 sessionmodification request and send the N4 session modification request to theUPF+PGW-U function network element.

S14. The user plane gateway receives the information related to the userplane tunnel of the voice service sent by the session management networkelement, and sets up the user plane tunnel for the voice service basedon the information related to the user plane tunnel of the voiceservice.

Steps S11 to S14 may be performed before step 702, or may be performedafter step 702 and before the user plane tunnel of the voice servicebegins to be switched. Therefore, the user plane tunnel that is set upby the user plane gateway for the voice service may also be switched tothe user plane tunnel of the voice service in the second network byswitching the user plane tunnel of the voice service, so that theterminal device may perform the voice service in the second network.

Manner 2. The session management network element is notified of reasoninformation indicating that setup of the user plane tunnel of the voiceservice is rejected, so that the session management network element setsup a user plane tunnel for the voice service of the terminal deviceafter the terminal device completes handover from the first network tothe second network.

Referring to FIG. 9, the manner 2 may specifically include the followingsteps.

S21. The first access network device sends third session managementinformation to the session management network element, where the thirdsession management information includes reason information indicatingthat setup of the user plane tunnel of the voice service of the terminaldevice is rejected.

Specifically, the NG RAN may add the third session managementinformation to an N2 session response and send the N2 session responseto the AMF, and then the AMF adds the third session managementinformation to a session management response with a PDU sessionmodification command and sends the session management response to theSMF+PGW-C function network element.

Optionally, the reason information indicating rejection may be that theterminal device is performing a handover process.

S22. The session management network element receives the third sessionmanagement information sent from the first access network device.

S23. The session management network element determines that the terminaldevice has handed over from a first network to a second network.

During specific implementation, step S23 may be implemented in either ofthe manner (1) and the manner (2). In the manner (1), a timer is set inthe session management network element, and when the timer times out, itis determined that the terminal device is handed over from the firstnetwork to the second network. In the manner (2), when a tunnelmodification request (modify bearer request) is received, the sessionmanagement network element determines that the terminal device is handedover from the first network to the second network, where the tunnelmodification request is for modifying signaling plane tunnel informationof the voice service of the terminal device.

S24. The session management network element sets up the user planetunnel for the voice service in the second network based on the reasoninformation.

During specific implementation of step S24, when the session managementnetwork element determines that the third session management informationincludes the reason information or the reason information is that theterminal device is performing a handover process, the user plane tunnelmay be set up for the voice service in the second network after theterminal device is handed over from the first network to the secondnetwork.

In the manner 2, although the network device in the first network doesnot set up a user plane tunnel for the voice service, after determiningthat the terminal device is handed over from the first network to thesecond network, the session management network element sets up the userplane tunnel for the voice service of the terminal device in the secondnetwork, so that the terminal device may perform the voice service inthe second network.

The manner 2 provides a method for performing network handover of theterminal device in a scenario in which the voice service cannot beperformed in the first network but the voice service may be performed inthe second network. When receiving the first session managementinformation, the first access network device can hand over the terminaldevice in the first network to the second network in time, so that theterminal device in the first network continues to perform a voice callsetup process in the second network. Moreover, the first access networkdevice rejects setup of the user plane tunnel of the voice servicebecause the reason information is that the terminal device is performinga handover process, so as to trigger the user plane gateway to set up auser plane tunnel of the voice service after the terminal device issuccessfully handed over to the second network (because a voice sessionmay be performed in the second network in the scenario, the user planetunnel may be set up successfully), so that the terminal device performsthe voice service in the second network.

Scenario 2. Neither the first network nor the voice service may beperformed in the second network.

In the scenario, the first condition may further include that the voiceservice cannot be performed in the second network.

In the scenario, the terminal device may perform network handover ineither of the following manner 3 and manner 4.

Manner 3. The session management network element is notified of reasoninformation indicating that setup of the user plane tunnel of the voiceservice is rejected, so that the session management network element setsup a user plane tunnel for the voice service of the terminal deviceafter the terminal device completes handover from the first network tothe second network, and when setup of the user plane tunnel fails, theterminal device is enabled to falls back to a 2G/3G network to performthe voice service.

The manner 3 may specifically include:

S31 to S34.

Steps S31 to S34 are respectively correspondingly the same as steps S21to S24, that is, step S31 is the same as step S21, step S32 is the sameas step S22, step S33 is the same as step S23, and step S34 is the sameas step S24. For details, refer to FIG. 9.

During specific implementation of step S34, a process of setting up theuser plane tunnel may be specifically implemented by using any one ofthe following processes P1 to P3.

Referring to FIG. 10, the process P1 may include the following steps.

(11). A serving gateway sends a tunnel setup request (create bearerrequest) to a second mobility management network element, where thetunnel setup request is for requesting to set up a user plane tunnel ofa voice service for a terminal device.

The serving gateway may be an SGW, the second mobility managementnetwork element may be an MME, and the user plane tunnel of the voiceservice may be a dedicated bearer whose QCI (the QCI is a quality ofservice class identifier, English full name qos class identifier) isequal to 1.

(12). The second mobility management network element receives the tunnelsetup request sent by the serving gateway, and sends a tunnel setupresponse (create bearer response) to the serving gateway based on asecond condition.

The tunnel setup response includes information indicating that setup ofthe user plane tunnel of the voice service of the terminal device isrejected.

The second condition includes that the voice service cannot be performedin the second network, and the second condition may further include thatthe terminal device is a terminal device handed over from the firstnetwork to the second network.

Specifically, when the second mobility management network elementdetermines that the second condition is satisfied, the second mobilitymanagement network element rejects setup of the user plane tunnel forthe voice service, and sends the tunnel setup response including theinformation to the serving gateway.

After step (12), the serving gateway may inform, through a sessionmanagement network element and a policy control function networkelement, an IMS network element of the information indicating that setupof the user plane tunnel of the voice service is rejected, so that theIMS network element triggers a circuit switched fallback (CSFB) serviceof the terminal device. The IMS network element may be a P-CSCF.

Referring to FIG. 11, the process P2 may include the following steps.

(21). A serving gateway sends a tunnel setup request to a secondmobility management network element, where the tunnel setup request isfor requesting to set up a user plane tunnel of a voice service for aterminal device.

(22). The second mobility management network element sends a radiotunnel setup request (bearer setup request) to a second access networkdevice, where the radio tunnel setup request is for requesting to set upthe user plane tunnel for the voice service of the terminal device.

The second access network device may be an eNB, and the user planetunnel of the voice service may be a dedicated bearer whose QCI is equalto 1.

(23). The second access network device receives the radio tunnel setuprequest sent by the second mobility management network element, andsends a radio tunnel setup response (bearer setup response) to thesecond mobility management network element based on a second condition.

The radio tunnel setup response includes information indicating thatsetup of the user plane tunnel of the voice service is rejected. Thesecond condition includes that the voice service cannot be performed inthe second network, and the second condition may further include thatthe terminal device is a terminal device handed over from the firstnetwork to the second network.

Specifically, when the second access network device determines that thesecond condition is satisfied, the second access network device rejectssetup of the user plane tunnel for the voice service, and sends theradio tunnel setup response including the information to the secondmobility management network element.

(24). The second mobility management network element receives the radiotunnel setup response sent by the second access network device and sendsa tunnel setup response to the serving gateway based on the secondcondition.

The tunnel setup response includes the information indicating that setupof the user plane tunnel of the voice service of the terminal device isrejected.

After step (24), the serving gateway may inform, through a sessionmanagement network element and a policy control function networkelement, an IMS network element of the information indicating that setupof the user plane tunnel of the voice service is rejected, so that theIMS network element triggers a CSFB service of the terminal device.

The policy control function network element may be a PCF+PCRF functionnetwork element.

Referring to FIG. 12, the process P3 may include the following steps.

(31). A serving gateway sends a tunnel setup request to a secondmobility management network element, where the tunnel setup request isfor requesting to set up a user plane tunnel of a voice service for aterminal device.

(32). The second mobility management network element receives the tunnelsetup request sent by the serving gateway and sends a radio tunnel setuprequest to a second access network device, where the radio tunnel setuprequest is for requesting to set up the user plane tunnel for the voiceservice of the terminal device.

(33). The second access network device receives the radio tunnel setuprequest sent by the second mobility management network element, andsends a radio tunnel setup response to the second mobility managementnetwork element based on a second condition.

The radio tunnel setup response includes information indicating thatsetup of the user plane tunnel of the voice service is rejected. Thesecond condition includes that the voice service cannot be performed inthe second network, and the second condition may further include thatthe terminal device is a terminal device handed over from the firstnetwork to the second network.

Specifically, when the second access network device determines that thesecond condition is satisfied, the second access network device rejectssetup of the user plane tunnel for the voice service, and sends theradio tunnel setup response including the information to the secondmobility management network element.

(34). The second mobility management network element receives the radiotunnel setup response sent by the second access network device and sendsa tunnel setup response to the serving gateway based on the radio tunnelsetup response.

The radio tunnel setup response includes the information indicating thatsetup of the user plane tunnel of the voice service of the terminaldevice is rejected, and the tunnel setup response includes theinformation indicating that setup of the user plane tunnel of the voiceservice of the terminal device is rejected.

After step (34), the serving gateway may inform, through a sessionmanagement network element and a policy control function networkelement, an IMS network element of the information indicating that setupof the user plane tunnel of the voice service is rejected, so that theIMS network element triggers a CSFB service of the terminal device.

Because a voice session cannot be performed in the second network in thescenario, the user plane tunnel of the voice service cannot be set upsuccessfully. In this case, the terminal device may be enabled to fallback to a 2G/3G network to perform the voice service. Therefore, theterminal device needs to perform a CSFB service. Joint registration is anecessary precondition on which the terminal device subsequentlyinitiates a CSFB call, and that the terminal device completes jointregistration means that the terminal device completes a jointregistration process of an EPS and a CS domain. Therefore, to ensurethat the terminal device successfully initiates the CSFB call, the CSFBservice of the terminal device needs to be triggered after the terminaldevice completes joint registration. This may be specificallyimplemented in any one of the following four manners W1 to W4.

Manner W1:

Referring to FIG. 10, in the process P1, step (12) is performed afterthe second mobility management network element determines that theterminal device completes joint registration. Specifically, when thesecond mobility management network element determines that a jointregistration complete message from the terminal device is received,where the joint registration complete message may be specifically atracking area update complete message, the second mobility managementnetwork element determines that the terminal device completes jointregistration.

Referring to FIG. 11, in the process P2, in step (24), after the secondmobility management network element determines that the second conditionis satisfied and the terminal device completes joint registration, thesecond mobility management network element sends the tunnel setupresponse to the serving gateway. Specifically, when the second mobilitymanagement network element determines that a joint registration completemessage from the terminal device is received, the second mobilitymanagement network element determines that the terminal device completesjoint registration.

Referring to FIG. 12, in the process P3, if the second condition issatisfied, step (32) is performed after the second mobility managementnetwork element determines that the terminal device completes jointregistration. Specifically, when the second mobility management networkelement determines that a joint registration complete message from theterminal device is received, the second mobility management networkelement determines that the terminal device completes jointregistration.

In the manner, because the IMS network element triggers, after receivingthe information indicating that setup of the user plane tunnel of thevoice service is rejected, the CSFB service of the terminal device, andthe second mobility management network element sends, after determiningthat the terminal device completes joint registration, the tunnel setupresponse to the serving gateway, the IMS network element triggers theCSFB service of the terminal device after the terminal device completesjoint registration, thereby ensuring that the terminal devicesuccessfully initiates the CSFB call.

Manner W2:

During specific implementation of step S34, the session managementnetwork element sets up the user plane tunnel for the voice service inthe second network after a first preset period based on the reasoninformation and a second condition.

Step S34 may be specifically implemented in a manner m1 and a manner m2.

The manner m1 is: sending, by the session management network element, atunnel setup request to a serving gateway of the second network afterthe first preset period based on the reason information and the secondcondition, where the tunnel setup request is for requesting to set upthe user plane tunnel of the voice service.

Specifically, a start time point of the first preset period may be atime point at which the reason information is received.

The manner m2 is: sending, by the session management network element, atunnel setup request to a serving gateway of the second network based onthe reason information, where the tunnel setup request is for requestingto set up the user plane tunnel of the voice service; receiving, by thesession management network element, a tunnel setup response sent fromthe serving gateway, where the tunnel setup response includesinformation indicating that setup of the user plane tunnel of the voiceservice is rejected; and sending, by the session management networkelement, a first message to a policy function control network elementafter the first preset period based on the second condition, where thefirst message is for indicating that setup of the user plane tunnel ofthe voice service fails. The first message may be a credit controlrequest (CCR), and information indicating that setup of the user planetunnel fails may be included in the CCR.

Specifically, a start time point of the first preset period may be atime point at which the reason information is received.

In the manner, it is considered that when the IMS network elementtriggers the CSFB service of the terminal device after a delay of thefirst preset period, the terminal device completes joint registration.Specifically, the session management network element may send the tunnelsetup request or the first message after a delay of the first presetperiod, so that the IMS network element triggers the CSFB service of theterminal device after a delay of the first preset period, therebyensuring that the terminal device successfully initiates the CSFB call.

Manner W3:

Referring to FIG. 13, the manner W3 may include the following steps.

W3-1. An IMS network element receives a request message for setting up avoice service for a terminal device in a packet switched (PS) domain.

The request message may be specifically an invite message.

W3-2. The IMS network element determines that a voice service cannot beset up in a PS domain of a first network or a second network.

W3-3. The IMS network element determines that the terminal devicecompletes joint registration.

W3-4. The IMS network element triggers a CSFB service of the terminaldevice.

In the manner, after determining that the terminal device completesjoint registration, the IMS network element triggers the CSFB service ofthe terminal device, thereby ensuring that the terminal devicesuccessfully initiates a CSFB call.

Optionally, after step W3-1, the method may further include:determining, by the IMS network element, based on the request messagefor setting up the voice service in the packet switched domain, that theterminal device is registered with the first network, and thendetermining that the request message is received in the first network.

Optionally, W3-2 may specifically include: obtaining, by the IMS networkelement, voice service setup failure information that is from the PSdomain of the first network, where specifically, when receivinginformation indicating that setup of the user plane tunnel of the voiceservice fails, the IMS network element determines, by determining thatthe terminal device is registered with the first network, that theinformation indicating that setup of the user plane tunnel of the voiceservice fails is from the PS domain of the first network; or obtaining,by the IMS network element, voice service setup failure information thatis from the PS domain of the second network, where specifically, whenreceiving information indicating that setup of the user plane tunnel ofthe voice service fails, the IMS network element determines, bydetermining that the terminal device is registered with the secondnetwork, that the information indicating that setup of the user planetunnel of the voice service fails is from the PS domain of the secondnetwork.

The voice service setup failure information may be reported by anothernetwork device to the IMS network element.

Optionally, W3-3 may specifically include: after a second preset periodsince the IMS network element determines that the voice service cannotbe set up in the PS domain of the first network or the second network,determining, by the IMS network element, that the terminal devicecompletes joint registration; or determining, by the IMS network elementbased on registration location information of the terminal deviceobtained from a policy control network element or a home subscriberserver (HSS), that the terminal device completes joint registration.

Specifically, the policy control network element or the home subscriberserver may actively report the registration location information of theterminal device to the IMS network element, or the registration locationinformation may be queried by the IMS network element from the policycontrol network element or the home subscriber server.

When the terminal device is a calling terminal device of the voiceservice, W3-4 may specifically include: sending, by the IMS networkelement, a second message to the calling terminal device, where thesecond message is for triggering the calling terminal device to initiatea CSFB calling voice service.

When the terminal device is a called terminal device of the voiceservice, W3-4 may specifically include: sending, by the IMS networkelement, a third message to a terminating access domain selectiondevice, where the third message is for indicating that setup of a calledvoice service in the PS domain of the first network or the secondnetwork fails, so that the terminating access domain selection devicetriggers a CSFB called voice service of the called terminal device.

The second message and the third message may be a 500 response message,a 380 response message, or a 503 response message.

Manner 4. The first access network device notifies the sessionmanagement network element of information indicating that setup of theuser plane tunnel of the voice service is rejected, and if theinformation indicating that setup of the user plane tunnel of the voiceservice is rejected includes that a radio resource is insufficient (orresource allocation fails), the session management network element doesnot set up the user plane tunnel for the voice service of the terminaldevice after the terminal device completes handover from the firstnetwork to the second network, and information indicating that setup ofthe user plane tunnel fails is transmitted to an IMS network through thefirst network, so that the IMS network triggers a CSFB service of theterminal device.

The manner 4 specifically includes: sending, by the first access networkdevice, fourth session management information to the session managementnetwork element, where the fourth session management informationincludes the information indicating that setup of the user plane tunnelof the voice service is rejected The information indicating that setupof the user plane tunnel of the voice service is rejected includes thata radio resource is insufficient.

In the manner, after the terminal device is handed over from the firstnetwork to the second network, the session management network elementdoes not set up a user plane tunnel for the voice service of theterminal device, but a handover process is improved, thereby ensuringthat the terminal device successfully initiates a CSFB call. This may bespecifically implemented in any one of W3 and the following manners W4to W7, where W5 and W6 are directed to a situation in which the terminaldevice is a calling terminal device.

Manner W4:

Referring to FIG. 14, the manner W4 includes the following steps.

W4-1. A second mobility management network element receives a fourthmessage sent by a first mobility management network element, where thefourth message is for determining that a terminal device completesrelocation.

The fourth message may be specifically a relocation request completeacknowledge message. Relocation herein means that in a process in whichthe terminal device is handed over from the first network to the secondnetwork, a mobility management network element corresponding to theterminal device is relocated, and is relocated from the first mobilitymanagement network element to the second mobility management networkelement.

W4-2. The second mobility management network element determines that thesecond condition is satisfied and the terminal device completes jointregistration.

The second condition includes that the voice service cannot be performedin the second network, and the second condition may further include thatthe terminal device is a terminal device handed over from the firstnetwork to the second network.

W4-3. The second mobility management network element sends a tunnelmodification request to a serving gateway, where the tunnel modificationrequest is for modifying signaling plane tunnel information of a voiceservice of the terminal device.

After receiving the tunnel modification request, the serving gatewaysends the tunnel modification request to a session management networkelement, and then the session management network element sends thetunnel modification request to a user plane gateway, so that the userplane gateway sets up a corresponding signaling plane tunnel of thevoice service based on the signaling plane tunnel information of thevoice service. The signaling plane tunnel of the voice service hereinmay be specifically a dedicated bearer whose QCI is equal to 5.

It should be noted that, after learning that setup of the user planetunnel fails, the IMS network element may send a second message (themeaning of the second message is the same as that mentioned above) tothe terminal device, and the second message needs to be transmitted tothe terminal device by using the signaling plane tunnel (that is, thededicated bearer whose QCI is equal to 5) of the voice service. Afterreceiving the tunnel modification request and completing setup of thesignaling plane tunnel of the voice service, the user plane gatewaysends the second message to the terminal device by using the signalingplane tunnel. In the manner, only after determining that the terminaldevice completes joint registration, the second mobility managementnetwork element sends the tunnel modification request to the servinggateway, thereby ensuring that the terminal device successfullyinitiates a CSFB call.

Manner W5:

Referring to FIG. 15, the manner W5 includes the following steps.

W5-1. An MME determines that a terminal device completes jointregistration.

W5-2. An IMS network element determines that a voice service cannot beset up in a PS domain of a first network or a second network.

W5-3. The IMS network element sends a second message to the terminaldevice.

For a method for implementing steps in the manner W5, refer to theforegoing description. Details are not described herein again.

In the manner, only after the MME determines that the terminal devicecompletes joint registration, the IMS network element sends the secondmessage to the terminal device. Therefore, it may be ensured that theterminal device successfully initiates a CSFB call.

Manner W6:

Referring to FIG. 16, the manner W6 includes the following steps.

W6-1. The IMS network element determines that a voice service cannot beset up in a PS domain of a first network or a second network.

W6-2. The IMS network element sends a second message to a user planegateway.

W6-3. The user plane gateway buffers the second message.

W6-4. An MME determines that a terminal device completes jointregistration.

W6-5. The MME sends a tunnel modification request to the user planegateway.

W6-6. After receiving the tunnel modification request, the user planegateway sends the second message to the terminal device.

For a method for implementing steps in the manner W6 and meanings ofrelated terms, refer to the foregoing description. Details are notdescribed herein again.

In the manner, the user plane gateway first buffers the second messagesent by the IMS network element, and after receiving the tunnelmodification request, the user plane gateway sends the second message tothe terminal device. The tunnel modification request is sent to the userplane gateway after the MME determines that the terminal devicecompletes joint registration. Therefore, it may be ensured that theterminal device successfully initiates a CSFB call.

Manner W7:

Referring to FIG. 17, the manner W7 includes the following steps.

W7-1. An IMS network element determines that a voice service cannot beset up in a PS domain of a first network or a second network.

W7-2. When a terminal device is a calling terminal device, the IMSnetwork element sends a second message to the terminal device after adelay of a second preset period; or when a terminal device is a calledterminal device, the IMS network element sends a third message to aterminating access domain selection device after a delay of a secondpreset period.

For a method for implementing steps in the manner W7 and meanings ofrelated terms, refer to the foregoing description. Details are notdescribed herein again.

In the manner, it is considered that when the IMS network elementtriggers a CSFB service of the terminal device after the delay of thesecond preset period, the terminal device completes joint registration,thereby ensuring that the terminal device successfully initiates a CSFBcall.

The first preset period and the second preset period may be preset basedon an actual network situation, and the first preset period may be thesame as or different from the second preset period. For example, thefirst preset period may be 500 ms, and the second preset period may be100 ms.

It should be noted that, a 5G network is unlike a 2G/3G network in whicha dedicated CS domain is deployed. Therefore, the 5G network cannotprovide a CS voice service similar to a conventional voice to theterminal device. In the manner 3 and the manner 4, the method providedin the embodiments of this application may further enable the terminaldevice to perform a CS voice service when the terminal device cannotperform an IMS voice service.

This application further provides the following four embodiments, todescribe the foregoing method in an example. Each of applicationscenarios of an embodiment (1) and an embodiment (2) is a scenario inwhich an IMS voice service cannot be performed in a 5G network but theIMS voice service may be performed in an EPS network, and each ofapplication scenarios of an embodiment (3) and an embodiment (4) is ascenario in which an IMS voice service cannot be performed in a 5Gnetwork and the IMS voice service cannot be performed in an EPS networkeither. In each embodiment, there may be a calling side and a calledside. Unless otherwise specified, UE and network elements in a methodfor describing the calling side are UE and network elements on thecalling side, and UE and network elements in a method for describing thecalled side are UE and network elements on the called side.

Embodiment (1)

This embodiment provides a method for performing network handover ofcalling UE. Referring to FIG. 18A and FIG. 18B, the method includes thefollowing steps.

1801. UE sends an invite message to a P-CSCF, where the invite messageis for requesting to set up an IMS voice service.

The UE is UE in a 5G network, and the invite message is a message forrequesting to set up, in a PS domain, a voice service for the UE in the5G network.

1802. The P-CSCF sets up an IMS voice session for the UE based on thereceived invite message sent by the UE.

The P-CSCF further sends the invite message to a node (for example, anIMS node such as an S-CSCF and a TAS) in a subsequent calling IMSnetwork, the node in the subsequent IMS network begins to process thereceived invite message, and called UE may receive the invite message ofthe calling UE forwarded by a called IMS network.

1803. The P-CSCF sends an authentication authorization request (AAR) toa PCF+PCRF to trigger setup of a user plane tunnel of the IMS voiceservice of the UE.

The AAR may carry a user identifier of the UE, an IMS application layercharging identifier, media description information of the IMS voiceservice, and the like. The media description information may include atleast one piece of the following information: a classifier identifier,bandwidth requirement description information, media type descriptioninformation, and a QoS parameter.

It should be noted that, the P-CSCF may directly send the AAR to thePCF+PCRF, or may send, after receiving a 183 response message sent bythe S-CSCF in the IMS network, the AAR to the PCF+PCRF.

1804. The PCF+PCRF sends a packet data unit-connectivity access network(PDU-CAN) session modification process request to an SMF+PGW-C based onmedia description information carried in the AAR sent by the P-CSCF, totrigger a network element in a 5G network to set up the user planetunnel for the IMS voice service of the UE.

The PDU-CAN session modification carries QoS rules, the QoS rulesinclude a QoS parameter set (profile), a packet filter, and the like,and the QoS parameter set usually includes one or more of the followingQoS key parameters: a GBR, an ARP, a 5QI, an MBR, and the like.

A user plane tunnel in the 5G network is usually a QoS flow used totransmit user plane data, and a 5QI of the user plane tunnel of the IMSvoice service may be equal to 1 or another value. As an example of themethod provided in this embodiment of this application, the 5QI of theuser plane tunnel of the IMS voice service is equal to 1.

1805. After receiving the PDU-CAN session modification process requestsent by the PCF+PCRF network element, the SMF+PGW-C sends a sessionmanagement request with a PDU session modification command to an AMF,where the request includes first SM information, the first SMinformation is SM information related to the user plane tunnel of theIMS voice service of the UE, the first SM information may include QoSparameters of the user plane tunnel, for example, parameters such as a5QI equal to 1, an ARP, a GBR, and an MBR, and the first SM informationmay further include other information.

1806. The AMF sends an N2 session request to an NG RAN based on thereceived first SM information sent by the SMF+PGW-C, where the first SMinformation is transparently transmitted to the NG RAN, so that the NGRAN allocates, based on the first SM information, a corresponding radioresource to the user plane tunnel whose 5QI is equal to 1.

1807. The NG RAN determines, based on the first SM information and afirst condition, not to allocate a radio resource to the user planetunnel of the IMS voice service of the UE.

It should be noted that, step 1807 may be performed or not performed.This is not specifically limited in this embodiment of this application.

The first condition may include the following conditions: (1) acondition that the IMS voice service cannot be performed in a 5Gnetwork, where the condition indicates that the IMS voice service is notdeployed in the 5G network, or signal coverage of the current 5G networkis too poor to satisfy a requirement of the IMS voice service; (2) acondition that an EPS network supports a voice service, where thecondition may be configured, as a local policy, in the NG RAN by anoperator; (3) a condition that signal quality of the EPS networksatisfies a requirement of the voice service, where the NG RAN maydetermine, by instructing the UE to measure and report signal quality ofa neighboring EPS cell, whether the signal quality of the EPS networksatisfies the requirement of the voice service; and (4) a condition thata network device (the AMF, the NG RAN, or the like) of the 5G networkand the UE support handover between the 5G network and the EPS network,where whether the UE and the AMF support handover between the 5G networkand the EPS network may be notified, by using an N2 message, to the NGRAN by the AMF when the UE is changed from an idle state to a connectedstate.

1808. The NG RAN sends an N2 session response to the AMF.

The N2 session response includes second SM information, and the secondSM information includes information indicating that setup of the userplane tunnel of the IMS voice service of the UE is accepted.

1809. The AMF sends a session management response with the PDU sessionmodification command to the SMF+PGW-C based on the received N2 sessionresponse, where the second SM information is transparently transmittedto the SMF+PGW-C.

1810. The SMF+PGW-C sends an N4 session modification request to aUPF+PGW-U based on the received session management response with the PDUsession modification command sent by the AMF. N4 herein indicates thatan N4 interface is between the SMF+PGW-C and the UPF+PGW-U, but a nameof the interface is not limited in this embodiment of this application,that is, a name of the message is not limited to the N4 sessionmodification request.

The N4 session modification request carries packet detection, anenforcement rule, a reporting rule, core network tunnel information, andthe like related to the user plane tunnel whose 5QI is equal to 1, sothat the UPF+PGW-U can set up a corresponding core network tunnel forthe user plane tunnel whose 5QI is equal to 1.

1811. The UPF+PGW-U receives the N4 session modification request sent bythe SMF+PGW-C, and sets up, based on the N4 session modificationrequest, a corresponding core network tunnel for the user plane tunnelwhose 5QI is equal to 1.

It should be noted that, a user plane tunnel in the 5G network isusually a QoS flow used to transmit user plane data, and a user planetunnel in the EPS network is usually a bearer used to transmit userplane data. After setting up the corresponding user plane tunnel for theuser plane tunnel whose 5QI is equal to 1, the UPF+PGW-U switches theuser plane tunnel in a process of handing over the UE from the 5Gnetwork to the EPS network, that is, switches the user plane tunnel froma QoS flow whose 5QI is equal to 1 to a dedicated bearer whose QCI isequal to 1. Specifically, the SMF+PGW-C may determine, by using areceived tunnel modification request sent by an SGW, that the user planetunnel whose 5QI is equal to 1 of the UE in the 5G network is switchedto a user plane tunnel whose QCI is equal to 1 in the EPS network, andthe user plane tunnel whose QCI is equal to 1 is set up successfully.After the UE is handed over from the 5G network to the EPS network, andthe UE receives related signaling of the IMS voice service forwarded bythe EPS network, the UE continues to perform a subsequent IMS voice callsetup process in the EPS network, thereby completely setting up the IMSvoice session.

After step 1811, the UPF+PGW-U further sends an N4 session modificationresponse to the SMF+PGW-C, the SMF+PGW-C sends the PDU-CAN sessionmodification to the PCF+PCRF, the PCF+PCRF sends a re-authenticationrequest (RAR) to the P-CSCF, and the P-CSCF sends a creditre-authentication answer (RAA) to the PCF+PCRF. Therefore, the P-CSCFcan learn that the user plane tunnel of the IMS voice service is set upsuccessfully.

1812. The NG RAN sends a handover request to the AMF based on thereceived first SM information and the first condition to trigger the UEto be handed over from the 5G network to the EPS network.

Specifically, when the first condition is satisfied, and the NG RANdetermines, based on the first SM information, that a user plane tunnelwhose 5QI is equal to 1 needs to be set up, the handover request may besent to the AMF.

For a handover process of handing over the UE from the 5G network to theEPS network, refer to a standard protocol. Details are not describedherein again. After the UE is handed over from the 5G network to the EPSnetwork, and the UE receives related signaling of the IMS voice serviceforwarded by the EPS network, the UE continues to perform a subsequentIMS voice call setup process in the EPS network, thereby completelysetting up the IMS voice session.

This embodiment further provides a method for performing networkhandover of called UE. Referring to FIG. 19A and FIG. 19B, the methodincludes the following steps.

1901. A P-CSCF receives an invite message sent by an S-CSCF, and sets upan IMS voice session for UE based on the invite message.

The invite message is a message for requesting to set up, in a PSdomain, a voice service for the UE in a 5G network.

1902. The P-CSCF sends the invite message to the UE.

1903. The UE receives the invite message sent by the P-CSCF and repliesto the P-CSCF with a 183 response message.

1904. The P-CSCF receives the 183 response message sent by the UE, andsends the 183 response message to the S-CSCF.

The S-CSCF may further send the 183 response message to a callingS-CSCF, the calling S-CSCF further sends the 183 response message to acalling P-CSCF, and if receiving the 183 response message sent by thecalling S-CSCF, the calling P-CSCF may forward the 183 response messageto calling UE.

1905 to 1914.

Steps 1905 to 1914 are respectively correspondingly the same as steps1803 to 1812. That is, step 1905 is the same as step 1803, step 1906 isthe same as step 1804, the rest may be deduced by analogy, step 1914 isthe same as step 1812, and the following similar description has a samemeaning.

Embodiment (1) provides a method for performing network handover of aterminal device in a scenario in which an IMS voice service cannot beperformed in a 5G network but the IMS voice service may be performed inan EPS network. When receiving a request for setting up a user planetunnel whose 5QI is equal to 1, the NG RAN can hand over a 5G terminalto the EPS network in time, so that the terminal device in 5G continuesto perform the IMS voice call setup process in the EPS network.

Embodiment (2)

This embodiment provides a method for performing network handover ofcalling UE. Referring to FIG. 20, the method includes the followingsteps.

2001 to 2006.

Steps 2001 to 2006 are respectively correspondingly the same as steps1801 to 1806.

2007. The NG RAN sends an N2 session response to the AMF based on thefirst SM information and a first condition.

The NG RAN may determine, based on the first SM information, that theuser plane tunnel of the IMS voice service that is specifically the userplane tunnel whose 5QI is equal to 1 is requested to be set up.

The N2 session response includes third SM information, the third SMinformation may include SM information indicating that the NG RANrejects allocation of a radio resource to the user plane tunnel whose5QI is equal to 1 and carry reason information indicating rejection, andthe reason information is that the UE is performing a handover process.Furthermore, the reason information may be that the UE is performinghandover from the 5G network to the EPS network, so that the SMF+PGW-Cdetermines, based on the reason information, to initiate setup of theuser plane tunnel of the IMS voice service of the UE after the handoverprocess of the UE ends.

The first condition may include the following conditions: (1) acondition that the IMS voice service cannot be performed in a 5Gnetwork, where the condition indicates that the IMS voice service is notdeployed in the 5G network, or signal coverage of the current 5G networkis too poor to satisfy a requirement of the IMS voice service; (2) acondition that an EPS network supports a voice service, where thecondition may be configured, as a local policy, in the NG RAN by anoperator; (3) a condition that signal quality of the EPS networksatisfies a requirement of the voice service, where the NG RAN maydetermine, by instructing the UE to measure and report signal quality ofa neighboring EPS cell, whether the signal quality of the EPS networksatisfies the requirement of the voice service; and (4) a condition thata network device (the AMF, the NG RAN, or the like) of the 5G networkand the UE support handover between the 5G network and the EPS network,where whether the UE and the AMF support handover between the 5G networkand the EPS network may be notified, by using an N2 message, to the NGRAN by the AMF when the UE is changed from an idle state to a connectedstate.

2008. The AMF sends the session management request with the PDU sessionmodification command to the SMF+PGW-C based on the received N2 sessionresponse, where the third SM information is transparently transmitted tothe SMF+PGW-C.

2009. The SMF+PGW-C determines, based on the third SM information in thereceived session management request with the PDU session modificationcommand sent by the AMF, to set up a user plane tunnel for the IMS voiceservice of the UE in an EPS network after the UE is handed over from the5G network to the EPS network.

It should be noted that, the setting up a user plane tunnel for the IMSvoice service of the UE in the EPS network is setting up a user planetunnel whose QCI is equal to 1.

In a process of handing over the UE from the 5G network to the EPSnetwork, the SMF+PGW-C may determine, by using a received tunnelmodification request sent by an SGW, that the UE is successfully handedover from the 5G network to the EPS network. Alternatively, theSMF+PGW-C may further set a timer, and trigger the timer at thebeginning of receiving the third SM information. If the timer times out,it is determined that the UE is successfully handed over from the 5Gnetwork to the EPS network.

2010. The NG RAN sends a handover request to the AMF based on thereceived first SM information and the first condition to trigger the UEto be handed over from the 5G network to the EPS network.

Specifically, when the first condition is satisfied, and the NG RANdetermines, based on the first SM information, that a user plane tunnelwhose 5QI is equal to 1 needs to be set up, the handover request may besent to the AMF.

For a handover process of handing over the UE from the 5G network to theEPS network, refer to a standard protocol. Details are not describedherein again.

2011. The SMF+PGW-C sends, after handover of the UE is completed, atunnel setup request (create bearer request) to the SGW to trigger setupof a user plane tunnel whose QCI is equal to 1.

Subsequently, the user plane tunnel whose QCI is equal to 1 issuccessfully set up in the EPS network, and the EPS network feeds back,to an IMS network, that the user plane tunnel of the IMS voice serviceis successfully set up. For details of a process of setting up the userplane tunnel whose QCI is equal to 1 and a process of setting up the IMSvoice session in the EPS network, refer to a standard protocol. Detailsare not described herein again.

This embodiment further provides a method for performing networkhandover of called UE. Referring to FIG. 21, the method includes thefollowing steps.

2101 to 2113.

Steps 2101 to 2104 are respectively correspondingly the same as steps1901 to 1904, and steps 2105 to 2113 are respectively correspondinglythe same as steps 2003 to 2111.

Embodiment (2) provides a method for performing network handover of aterminal device in a scenario in which an IMS voice service cannot beperformed in a 5G network but the IMS voice service may be performed inan EPS network. When receiving a request for setting up a user planetunnel whose 5QI is equal to 1, the NG RAN can hand over a 5G terminalto the EPS network in time, so that the terminal device in 5G continuesto perform the IMS voice call setup process in the EPS network.Moreover, the NG RAN rejects setup of the user plane tunnel whose 5QI isequal to 1 based on reason information being that handover is beingperformed, so as to trigger a gateway of a core network to set up theuser plane tunnel whose QCI is equal to 1 after the UE is successfullyhanded over to the EPS network.

Embodiment (3)

This embodiment provides a method for performing network handover ofcalling UE. Referring to FIG. 22A and FIG. 22B, the method includes thefollowing steps.

2201 to 2210.

Steps 2201 to 2210 are respectively correspondingly the same as steps2001 to 2010.

2211. The SMF+PGW-C sends, after handover of the UE is completed, atunnel setup request to the SGW to trigger setup of a user plane tunnelwhose QCI is equal to 1.

2212. The SGW forwards the tunnel setup request to an MME.

After step 2212, the MME receives the tunnel setup request and performsany one of a process 1 (which includes a1), a process 2 (which includesb1 to b4), and a process 3 (which includes c1 to c4) in the followingprocesses.

Process 1:

a1. The MME sends a tunnel setup response to the SGW based on a secondcondition, where the tunnel setup response includes informationindicating that setup of the user plane tunnel of the voice service ofthe UE is rejected.

In the process 1, the second condition may include that the voiceservice cannot be performed in the second network, and may furtherinclude at least one of that the UE is UE handed over from the 5Gnetwork to the EPS network and that the UE completes joint registration,where that the voice service cannot be performed in the second networkmay specifically mean that the IMS voice service cannot be performed inthe second network, and that the UE completes joint registration meansthat the UE completes a joint registration process of an EPS and a CSdomain.

Specifically, the MME may determine, based on a received registrationcomplete message sent by the UE, that the UE completes jointregistration.

Before a1, the MME may send a radio tunnel setup request to an eNB forthe IMS voice service and receive a response message, or may neithersend a radio tunnel setup request nor receive a response message. Thisis not limited herein.

Process 2:

b1. The MME sends a radio tunnel setup request to an eNB, where theradio tunnel setup request is for requesting to set up the user planetunnel for the IMS voice service of the UE. Specifically, the user planetunnel of the IMS voice service may be a dedicated bearer whose QCI isequal to 1.

b2. The eNB receives the radio tunnel setup request, and sends a radiotunnel setup response to the MME based on a second condition, where theradio tunnel setup response includes information indicating that setupof the user plane tunnel of the IMS voice service of the UE is rejected.

In the process 2, the second condition includes that the IMS voiceservice cannot be performed in the second network, and may furtherinclude that the UE is UE handed over from the 5G network to the EPSnetwork.

b3. The MME receives the radio tunnel setup response sent by the eNB,and if the second condition is satisfied, the MME determines that the UEcompletes joint registration.

The radio tunnel setup response includes the information indicating thatsetup of the user plane tunnel of the IMS voice service of the UE isrejected.

Specifically, during specific implementation of step b3, when the MMEdetermines that a registration complete message sent by the UE isreceived, the MME determines that the UE completes joint registration.

b4. The MME sends a tunnel setup response to the SGW, where the tunnelsetup response includes the information indicating that setup of theuser plane tunnel of the IMS voice service of the UE is rejected.

Process 3:

c1. If a second condition is satisfied, the MME determines that the UEcompletes joint registration.

Specifically, during specific implementation of step c1, when the MMEdetermines that a registration complete message sent by the UE isreceived, the MME determines that the UE completes joint registration.

In the process 3, the second condition includes that the IMS voiceservice cannot be performed in the second network, and may furtherinclude that the UE is UE handed over from the 5G network to the EPSnetwork.

c2. The MME sends a radio tunnel setup request to an eNB, where theradio tunnel setup request is for requesting to set up the user planetunnel for the IMS voice service of the UE.

c3. The eNB receives the radio tunnel setup request sent by the MME, andsends a radio tunnel setup response to the MME based on a secondcondition, where the radio tunnel setup response includes informationindicating that setup of the user plane tunnel of the IMS voice serviceof the UE is rejected.

c4. The MME receives the radio tunnel setup response sent by the eNB,and sends a tunnel setup response to the SGW based on the radio tunnelsetup response, where the tunnel setup response includes the informationindicating that setup of the user plane tunnel of the IMS voice serviceof the UE is rejected.

2213. The SGW receives a tunnel setup response sent by the MME, andsends the tunnel setup response to the SMF+PGW-C.

2214. The SMF+PGW-C sends a CCR to a PCF+PCRF.

The CCR carries an indication for indicating that setup of the userplane tunnel whose QCI is equal to 1 fails, and may further carry reasoninformation indicating that setup of the tunnel fails, and the reasoninformation is specifically set with reference to reason informationcarried in the tunnel setup response sent by the SGW. The CCR herein maybe further replaced with a message having another name, for example, anIP-CAN session modification request. This is not limited herein.

2215. The PCF+PCRF sends a credit control answer (CCA) to the SMF+PGW-C.

2216. The PCF+PCRF sends an RAR to a P-CSCF based on the CCR, where theRAR may carry indication information for indicating that a radioresource is insufficient (or indication information for indicating thatresource allocation fails).

2217. The P-CSCF sends an RAA to the PCF+PCRF.

2218. The P-CSCF determines, based on the obtained indicationinformation indicating that the radio resource is insufficient (orindication information indicating that resource allocation fails), thatthe IMS voice service of the UE cannot be set up in a PS domain.

2219. The P-CSCF sends a 500 response message to the UE.

After step 2219, after receiving the 500 response message sent by theP-CSCF, the UE may perform calling domain selection and initiate a CSFBcall, a UE side cooperates with a network side to perform a CSFBprocess, and finally a voice call may be set up in a CS domain.

After step 2219, if the P-CSCF receives a 183 response message, theP-CSCF may send a cancel message to a node in an IMS network such as anS-CSCF, to cancel a requested call related to the IMS voice service.

It should be noted that, in a process of handing over the UE from the 5Gnetwork to the EPS network, a mobility management network elementserving the UE is changed from the AMF to the MME. Therefore, the MMEallocates a new globally unique temporary identity (GUTI) to the UE.Referring to FIG. 22A and FIG. 22B, after the handover process, the UEsends a tracking area update (TAU) request to the MME, and whenreceiving the TAU request sent by the NG RAN, the MME provides theallocated new GUTI to the UE by using a TAU accept message, so thatafter receiving the TAU accept message carrying the new GUTI, the UEreplies to the MME with a TAU complete message (that is, a jointregistration complete message).

In the foregoing description of this embodiment, only after determiningthat the UE completes the joint registration process of the EPS and theCS domain, the MME sends the tunnel setup response to the SGW, aiming toensure that a message indicating that setup of the user plane tunnelwhose QCI is equal to 1 fails reaches the IMS network only after the UEcompletes the joint registration process of the EPS and the CS domain,so that the 500 response message sent by the IMS network may reach theUE only after the UE completes the joint registration process of the EPSand the CS domain, to finally ensure that the UE successfully initiatesthe CSFB call.

To ensure that the 500 response message sent by the IMS network mayreach the UE only after the UE completes the joint registration processof the EPS and the CS domain, in addition to enabling the MME to send,only after determining that the UE completes the joint registrationprocess of the EPS and the CS domain, the tunnel setup response to theSGW, any one of the following manner 1 to manner 5 may be implemented.

Manner 1. The SMF+PGW-C sends the tunnel setup request to the SGW aftera delay of a first preset period based on the second condition.

In the manner 1, the second condition includes that the IMS voiceservice cannot be performed in the EPS network.

Manner 2. The SMF+PGW-C sends the CCR to the PCF+PCRF after a delay of afirst preset period.

Manner 3. After obtaining the indication information indicating that theradio resource is insufficient or the indication information indicatingthat resource allocation fails, the P-CSCF sends the 500 responsemessage to the UE after a delay of a second preset period.

Manner 4. After the SMF+PGW-C sends the CCR to the PCF+PCRF, theSMF+PGW-C instructs the UPF+PGW-U to buffer, for a first preset period,data that is on a user plane tunnel whose QCI is equal to 5 and that isto be sent to the UE, and after the first preset period, the UPF+PGW-Umay continue to send the data to the UE.

Manner 5. After determining that the UE completes joint registration,the P-CSCF sends the 500 response message to the UE.

During specific implementation, the manner 5 may include: after a secondpreset period since the P-CSCF determines that a voice service cannot beset up in the PS domain of the 5G network or the EPS network,determining, by the P-CSCF, that the terminal device completes jointregistration; or determining, by the P-CSCF based on registrationlocation information of the terminal device obtained from an HSS/homelocation register (HLR), that the terminal device completes jointregistration.

The first preset period and the second preset period may be set based onan actual network communication condition. For example, the first presetperiod and the second preset period may each be 500 ms.

This embodiment further provides a method for performing networkhandover of called UE. Referring to FIG. 23A and FIG. 23B, the methodincludes the following steps.

2301 to 2304.

Steps 2301 to 2304 are respectively correspondingly the same as steps1901 to 1904.

2305 to 2320.

Steps 2305 to 2320 are respectively correspondingly the same as steps2203 to 2218.

2321. The P-CSCF sends a 500 response message to a TAS in an IMS networkthrough the S-CSCF based on the obtained indication informationindicating that the radio resource is insufficient (or indicationinformation indicating that resource allocation fails), where the 500response message is specifically sent to a terminating access domainselection function module T-ADS in the TAS.

2322. After obtaining the indication information indicating that theradio resource is insufficient (or the indication information indicatingthat resource allocation fails), the P-CSCF sends a cancel message tothe UE, to cancel a requested session related to the IMS voice service.

A precedence relationship between step 2321 and step 2322 is notlimited.

After step 2322, the TAS in the IMS network reselects a CS domain as adomain of a called session, and initiates a call to a CS domain callcontrol device (for example, a mobile switching center (MSC)) through adevice such as the S-CSCF, the CS domain call control device initiates acall to the MME through an SGs interface, the MME triggers a CSFBprocess of the UE, and the UE may finally set up a voice session in theCS domain by using the CSFB process of the UE. For details of thespecific process, refer to related description in 3GPP TS 23.272.Details are not described herein again.

In the foregoing description of this embodiment, only after determiningthat the UE completes the joint registration process of the EPS and theCS domain, the MME sends the tunnel setup response to the SGW, aiming toensure that a message indicating that setup of the user plane tunnelwhose QCI is equal to 1 fails reaches the IMS network only after the UEcompletes the joint registration process of the EPS and the CS domain,so that the 500 response message sent by the IMS network may reach theTAS only after the UE completes the joint registration process of theEPS and the CS domain, to finally ensure that an IMS called voiceservice may be converted into a CSFB called voice service that is toreach the UE.

To ensure that the 500 response message sent by the IMS network mayreach the TAS only after the UE completes the joint registration processof the EPS and the CS domain, in addition to enabling the MME to send,only after determining that the UE completes the joint registrationprocess of the EPS and the CS domain, the tunnel setup response to theSGW, any one of the following manner 1 to manner 5 may be implemented.

Manner 1. The SMF+PGW-C sends the tunnel setup request to the SGW aftera delay of a first preset period based on a second condition.

In the manner 1, the second condition includes that the IMS voiceservice cannot be performed in the EPS network.

Manner 2. The SMF+PGW-C sends the CCR to the PCF+PCRF after a delay of afirst preset period.

Manner 3. After obtaining the indication information indicating that theradio resource is insufficient (or the indication information indicatingthat resource allocation fails), the P-CSCF sends the 500 responsemessage to the TAS after a delay of a second preset period.

Manner 4. After the SMF+PGW-C sends the CCR to the PCF+PCRF, theSMF+PGW-C instructs the UPF+PGW-U to buffer, for a first preset period,data (which may be the cancel message) that is on a user plane tunnelwhose QCI is equal to 5 and that is to be sent to the UE, and after thefirst preset period, the UPF+PGW-U may continue to send the data to theUE.

Manner 5. After determining that the UE completes joint registration,the P-CSCF sends the 500 response message to the TAS.

During specific implementation, the manner 5 may include: after a secondpreset period since the P-CSCF determines that a voice service cannot beset up in the PS domain of the 5G network or the EPS network,determining, by the P-CSCF, that the terminal device completes jointregistration; or determining, by the P-CSCF based on registrationlocation information of the terminal device obtained from an HSS/HLR,that the terminal device completes joint registration.

The first preset period and the second preset period may be set based onan actual network communication condition. For example, the first presetperiod and the second preset period may each be 500 ms.

Embodiment (4)

This embodiment provides a method for performing network handover ofcalling UE. Referring to FIG. 24, FIG. 25, FIG. 26, and FIG. 27, themethod includes the following steps.

2401 to 2406.

Steps 2401 to 2406 are respectively correspondingly the same as steps1801 to 1806.

A scenario to which this embodiment is applied is a scenario in which anIMS voice service cannot be performed in the 5G network and the IMSvoice service cannot be performed in an EPS network either. Therefore,to ensure that the UE successfully initiates a CSFB call, a 500 responsemessage sent by an IMS network needs to reach the UE only after the UEcompletes a joint registration process of an EPS and a CS domain.

To ensure that the UE successfully initiates the CSFB call, after step2406, network handover of the UE may be completed by using any one ofthe following process 1 (referring to FIG. 24), process 2 (referring toFIG. 25), process 3 (referring to FIG. 26), and process 4 (referring toFIG. 27).

Process 1:

A1. The NG RAN sends a handover request to the AMF based on the receivedfirst SM information and a first condition to trigger the UE to behanded over from the 5G network to the EPS network.

In this case, the first condition may include the following conditions:(1) a condition that the IMS voice service cannot be performed in a 5Gnetwork, where the condition indicates that the IMS voice service is notdeployed in the 5G network, or signal coverage of the current 5G networkis too poor to satisfy a requirement of the IMS voice service; (2) acondition that a network device (the AMF, the NG RAN, or the like) ofthe 5G network and the UE support handover between the 5G network andthe EPS network, where whether the UE and the AMF support handoverbetween the 5G network and the EPS network may be notified, by using anN2 message, to the NG RAN by the AMF when the UE is changed from an idlestate to a connected state; (3) a condition that an EPS network does notsupport a voice service, where the condition may be configured, as alocal policy, in the NG RAN by an operator; and (4) a condition thatsignal quality of the EPS network does not satisfy a requirement of thevoice service, where the NG RAN may determine, by instructing the UE tomeasure and report signal quality of a neighboring EPS cell, whether thesignal quality of the EPS network satisfies the requirement of the voiceservice.

A2. In a process of handing over the UE from the 5G network to the EPSnetwork, when a handover preparation process ends, the NG RAN sends ahandover command to the UE.

A3. The NG RAN sends an N2 session response to the AMF based on thefirst condition, where the N2 session response carries fourth SMinformation, the fourth SM information includes information indicatingthat setup of the user plane tunnel of the IMS voice service isrejected, and the information indicating that setup of the user planetunnel of the IMS voice service is rejected includes that the radioresource is insufficient (or that resource allocation fails).

A4. The AMF sends a session management response with the PDU sessionmodification command to the SMF+PGW-C based on the received N2 sessionresponse, where the fourth SM information is transparently transmittedto the SMF+PGW-C.

A5. After parsing the fourth SM information, the SMF+PGW-C sends aPDU-CAN session modification message to the PCF+PCRF.

The PDU-CAN session modification message includes information forindicating that setup of the user plane tunnel of the IMS voice servicefails, and the indication information may be indication informationindicating that the radio resource is insufficient (or indicationinformation indicating that resource allocation fails). In thisembodiment, a name of the message is not limited to the PDU-CAN sessionmodification message, and may be another message name such as CCR.

A6. The PCF+PCRF sends an RAR to the P-CSCF, where the RAR may carry theinformation for indicating that setup of the user plane tunnel of theIMS voice service fails.

A7. The P-CSCF determines, based on the received information forindicating that setup of the user plane tunnel fails, that the IMS voiceservice cannot be set up in a PS domain.

A8. The P-CSCF sends a 500 response message to the UE, where the messagereaches a UPF+PGW-U.

The 500 response message is transmitted to the UE by using a tunnel(which is specifically a QoS flow) of the 5G network.

A9. In the process of handing over the UE from the 5G network to the EPSnetwork, an MME determines that the UE completes joint registration.

A10. The MME sends a tunnel modification request for a user plane tunnelwhose QCI is equal to 5 to the SMF+PGW-C through an SGW, to indicate adownlink SGW user plane address and tunnel information of a dedicatedbearer whose QCI is equal to 5 to the SMF+PGW-C.

A11. The SMF+PGW-C sends the tunnel modification request to theUPF+PGW-U, to indicate the downlink SGW user plane address and thetunnel information of the dedicated bearer whose QCI is equal to 5 tothe UPF+PGW-U. After this, the SMF+PGW-C feeds back a tunnelmodification response to the MME through the SGW.

A12. The UPF+PGW-U forwards user plane data on the dedicated bearerwhose QCI is equal to 5 to the UE, that is, the 500 response message issent to the UE.

After step A12, the UE initiates a CSFB calling voice service, a UE sidecooperates with a network side to perform a CSFB process, and finally avoice call may be set up in the CS domain.

In this case, the UE leaves the 5G network and accesses the EPS network.Therefore, after reaching the UPF+PGW-U, the 500 response message issent to the UE by the UPF+PGW-U through an IMS signaling tunnel (theuser plane tunnel whose QCI is equal to 5) of the EPS network. If theP-CSCF receives a 183 response message, the P-CSCF may send a cancelmessage to a node in an IMS network such as an S-CSCF, to cancel arequested call related to the IMS voice service.

In the process 1, only after receiving the tunnel modification request,the UPF+PGW-U sends the 500 response message to the UE. Therefore, afterdetermining that the UE completes joint registration, the MME sends thetunnel modification request for the user plane tunnel whose QCI is equalto 5 to the SMF+PGW-C and then to the UPF+PGW-U, so that the 500response message sent by the IMS network reaches the UE only after theUE completes the joint registration process of the EPS and the CSdomain, to ensure that the UE successfully initiates the CSFB call.

Process 2:

B1. The NG RAN sends an N2 session response to the AMF based on a firstcondition, where the N2 session response carries fourth SM information,the fourth SM information includes information indicating that setup ofthe user plane tunnel of the IMS voice service is rejected, and theinformation indicating that setup of the user plane tunnel of the IMSvoice service is rejected includes that the radio resource isinsufficient (or that resource allocation fails).

Specifically, the NG RAN may determine the fourth SM information basedon the first condition.

In this case, the first condition may include the following conditions:(1) a condition that the IMS voice service cannot be performed in a 5Gnetwork, where the condition indicates that the IMS voice service is notdeployed in the 5G network, or signal coverage of the current 5G networkis too poor to satisfy a requirement of the IMS voice service; (2) acondition that a network device (the AMF, the NG RAN, or the like) ofthe 5G network and the UE support handover between the 5G network andthe EPS network, where whether the UE and the AMF support handoverbetween the 5G network and the EPS network may be notified, by using anN2 message, to the NG RAN by the AMF when the UE is changed from an idlestate to a connected state; (3) a condition that an EPS network does notsupport a voice service, where the condition may be configured, as alocal policy, in the NG RAN by an operator; and (4) a condition thatsignal quality of the EPS network does not satisfy a requirement of thevoice service, where the NG RAN may determine, by instructing the UE tomeasure and report signal quality of a neighboring EPS cell, whether thesignal quality of the EPS network satisfies the requirement of the voiceservice.

B2. The AMF sends a session management response with the PDU sessionmodification command to the SMF+PGW-C based on the received N2 sessionresponse, where the fourth SM information is transparently transmittedto the SMF+PGW-C.

B3. The SMF+PGW-C determines to retain notifying the PCF+PCRF, that is,postpones notifying the PCF+PCRF of the information indicating thatsetup of the user plane tunnel of the voice service is rejected.

B4. The NG RAN sends a handover request to the AMF based on the firstcondition and the first SM information.

B5. In a handover process, the MME determines that the UE completesjoint registration.

B6. The MME sends a tunnel modification request for a user plane tunnelwhose QCI is equal to 5 to the SMF+PGW-C through an SGW, to indicate adownlink SGW user plane address and tunnel information to the SMF+PGW-C.

B7. The SMF+PGW-C sends the tunnel modification request to theUPF+PGW-U, to indicate the downlink SGW user plane address and thetunnel information of the dedicated bearer whose QCI is equal to 5 tothe UPF+PGW-U. After this, the SMF+PGW-C feeds back a tunnelmodification response to the MME through the SGW.

B8. The SMF+PGW-C sends an IP-CAN session modification message to thePCF+PCRF.

The IP-CAN session modification message carries information forindicating that setup of the user plane tunnel of the IMS voice servicefails, and the indication information may be indication informationindicating that the radio resource is insufficient (or indicationinformation indicating that resource allocation fails). In thisembodiment, a name of the message is not limited to the IP-CAN sessionmodification message, and may be another message name such as CCR.

B9. The PCF+PCRF sends an RAR to the P-CSCF, where the RAR carries theinformation for indicating that setup of the user plane tunnel of theIMS voice service fails.

B10. The P-CSCF determines, based on the received information forindicating that setup of the user plane tunnel fails, that the IMS voiceservice cannot be set up in a PS domain.

B11. The P-CSCF sends a 500 response message to the UE.

After step B11, the UE initiates a CSFB calling voice service, a UE sidecooperates with a network side to perform a CSFB process, and finally avoice call may be set up in the CS domain.

In the process 2, only after determining that setup of the user planetunnel of the IMS voice service fails, the P-CSCF sends the 500 responsemessage to the UE. Therefore, after the MME determines that the UEcompletes joint registration, the PCF+PCRF sends, to the P-CSCF, theinformation for indicating that setup of the user plane tunnel of theIMS voice service fails, so that the 500 response message sent by theIMS network reaches the UE only after the UE completes the jointregistration process of the EPS and the CS domain, to ensure that the UEsuccessfully initiates the CSFB call.

Process 3:

C1. The NG RAN sends an N2 session response to the AMF based on a firstcondition, where the N2 session response carries fourth SM information,the fourth SM information includes information indicating that setup ofthe user plane tunnel of the IMS voice service is rejected, and theinformation indicating that setup of the user plane tunnel of the IMSvoice service is rejected includes that the radio resource isinsufficient (or that resource allocation fails).

Specifically, the NG RAN may determine the fourth SM information basedon the first condition.

In this case, the first condition may include the following conditions:(1) a condition that the IMS voice service cannot be performed in a 5Gnetwork, where the condition indicates that the IMS voice service is notdeployed in the 5G network, or signal coverage of the current 5G networkis too poor to satisfy a requirement of the IMS voice service; (2) acondition that a network device (the AMF, the NG RAN, or the like) ofthe 5G network and the UE support handover between the 5G network andthe EPS network, where whether the UE and the AMF support handoverbetween the 5G network and the EPS network may be notified, by using anN2 message, to the NG RAN by the AMF when the UE is changed from an idlestate to a connected state; (3) a condition that an EPS network does notsupport a voice service, where the condition may be configured, as alocal policy, in the NG RAN by an operator; and (4) a condition thatsignal quality of the EPS network does not satisfy a requirement of thevoice service, where the NG RAN may determine, by instructing the UE tomeasure and report signal quality of a neighboring EPS cell, whether thesignal quality of the EPS network satisfies the requirement of the voiceservice.

C2. The AMF sends a session management response with the PDU sessionmodification command to the SMF+PGW-C based on the received N2 sessionresponse, where the fourth SM information is transparently transmittedto the SMF+PGW-C.

C3. The SMF+PGW-C sends an N4 session modification request to aUPF+PGW-U based on the received session management response with the PDUsession modification command sent by the AMF.

The N4 session modification request includes a first command, and thefirst command is for instructing the UPF+PGW-U to buffer data on a userplane tunnel whose 5QI is equal to 5.

C4. The UPF+PGW-U feeds back an N4 session modification response to theSMF+PGW-C based on the received N4 session modification request.

C5. The SMF+PGW-C sends a PDU-CAN session modification message to thePCF+PCRF.

The PDU-CAN session modification message carries information forindicating that setup of the user plane tunnel of the IMS voice servicefails, and the indication information may be indication informationindicating that the radio resource is insufficient (or indicationinformation indicating that resource allocation fails). In thisembodiment, a name of the message is not limited to the IP-CAN sessionmodification message, and may be another message name such as CCR.

C6. The PCF+PCRF sends an RAR to the P-CSCF, where the RAR carries theinformation for indicating that setup of the user plane tunnel of theIMS voice service fails.

C7. The P-CSCF feeds back an RAA to the PCF+PCRF.

C8. The P-CSCF determines, based on the received information forindicating that setup of the user plane tunnel fails, that the IMS voiceservice cannot be set up in a PS domain.

C9. The P-CSCF sends a 500 response message to the UE, where the messagereaches a UPF+PGW-U.

C10. The UPF+PGW-U buffers data on a user plane tunnel whose 5QI isequal to 5.

The data on a user plane tunnel whose 5QI is equal to 5 may include the500 response message.

C11. The NG RAN sends a handover request to the AMF based on the firstcondition and the first SM information.

C12. The MME determines that the UE completes joint registration.

C13. The MME sends a tunnel modification request (modify bearer request)for a user plane tunnel whose QCI is equal to 5 to the SMF+PGW-C throughan SGW, to indicate a downlink SGW user plane address and tunnelinformation to the SMF+PGW-C.

C14. The SMF+PGW-C sends the tunnel modification request to theUPF+PGW-U, where the tunnel modification request includes a secondcommand, and the second command is for instructing the UPF+PGW-U to stopbuffering information about the data on the user plane tunnel whose QCIis equal to 1.

After step C14, the UPF+PGW-U sends the 500 response message to the UE,the UE initiates a CSFB calling voice service, a UE side cooperates witha network side to perform a CSFB process, and finally a voice call maybe set up in the CS domain.

In the process 3, the 500 response message sent by the P-CSCF isbuffered in the UPF+PGW-U by using the first command, and after the MMEdetermines that the UE completes joint registration, the P-CSCF sendsthe 500 response message to the terminal device by using the secondcommand, so that the 500 response message sent by the IMS networkreaches the UE only after the UE completes the joint registrationprocess of the EPS and the CS domain, to ensure that the UE successfullyinitiates the CSFB call.

Process 4:

D1. The NG RAN sends an N2 session response to the AMF based on a firstcondition, where the N2 session response carries fourth SM information,the fourth SM information includes information indicating that setup ofthe user plane tunnel of the IMS voice service is rejected, and theinformation indicating that setup of the user plane tunnel of the IMSvoice service is rejected includes that the radio resource isinsufficient (or that resource allocation fails).

Specifically, the NG RAN may determine the fourth SM information basedon the first condition.

In this case, the first condition may include the following conditions:(1) a condition that the IMS voice service cannot be performed in a 5Gnetwork, where the condition indicates that the IMS voice service is notdeployed in the 5G network, or signal coverage of the current 5G networkis too poor to satisfy a requirement of the IMS voice service; (2) acondition that a network device (the AMF, the NG RAN, or the like) ofthe 5G network and the UE support handover between the 5G network andthe EPS network, where whether the UE and the AMF support handoverbetween the 5G network and the EPS network may be notified, by using anN2 message, to the NG RAN by the AMF when the UE is changed from an idlestate to a connected state; (3) a condition that an EPS network does notsupport a voice service, where the condition may be configured, as alocal policy, in the NG RAN by an operator; and (4) a condition thatsignal quality of the EPS network does not satisfy a requirement of thevoice service, where the NG RAN may determine, by instructing the UE tomeasure and report signal quality of a neighboring EPS cell, whether thesignal quality of the EPS network satisfies the requirement of the voiceservice.

D2. The AMF sends a session management response with the PDU sessionmodification command to the SMF+PGW-C based on the received N2 sessionresponse, where the fourth SM information is transparently transmittedto the SMF+PGW-C.

D3. The SMF+PGW-C sends a PDU CAN session modification message to thePCF+PCRF. In this embodiment, a name of the message is not limited tothe IP-CAN session modification message, and may be another message namesuch as CCR.

The PDU CAN session modification message carries information forindicating that setup of the user plane tunnel of the IMS voice servicefails, and the indication information may be indication informationindicating that the radio resource is insufficient (or indicationinformation indicating that resource allocation fails).

D4. The PCF+PCRF sends an RAR to the P-CSCF, where the RAR carries theinformation for indicating that setup of the user plane tunnel of theIMS voice service fails.

D5. The P-CSCF determines, based on the received information forindicating that setup of the user plane tunnel fails, that the IMS voiceservice cannot be set up in a PS domain.

D6. The P-CSCF feeds back an RAA message to the PCF+PCRF.

D7. The P-CSCF determines to send a 500 response message after a delayof a second preset period.

D8. The NG RAN sends a handover request to the AMF based on the receivedfirst SM information and the first condition to trigger the UE to behanded over from the 5G network to the EPS network.

After step D8, the UE performs a handover process. After the handoverprocess ends, the P-CSCF sends the 500 response message to the UE, theUE initiates a CSFB calling voice service, a UE side cooperates with anetwork side to perform a CSFB process, and finally a voice call may beset up in the CS domain.

In the process 4, the P-CSCF sends the 500 response message after thedelay of the second preset period, so that the 500 response message sentby the IMS network reaches the UE only after the UE completes the jointregistration process of the EPS and the CS domain, to ensure that the UEsuccessfully initiates the CSFB call.

This embodiment further provides a method for performing networkhandover of called UE. A scenario to which this embodiment is applied isa scenario in which an IMS voice service cannot be performed in the 5Gnetwork and the IMS voice service cannot be performed in an EPS networkeither. Therefore, to ensure that the UE successfully initiates a CSFBcall, a 500 response message sent by an IMS network needs to reach theTAS only after the UE completes a joint registration process of an EPSand a CS domain, so that the TAS reselects a CS domain as a domain ofthe called UE, to ensure that the CSFB call succeeds. Referring to FIG.28, the method includes the following steps.

2801 to 2808.

Steps 2801 to 2808 are respectively correspondingly the same as steps1901 to 1908.

2809 to 2814.

Steps 2809 to 2814 are respectively correspondingly the same as steps D1to D6.

2815. The P-CSCF sends a 500 response message to a TAS after a delay ofa second preset period. The 500 response message is specifically sent toa terminating access domain selection function module T-ADS in the TAS.

The P-CSCF sends the 500 response message after the delay of the secondpreset period, so that the 500 response message sent by the P-CSCFreaches the TAS only after the UE completes the joint registrationprocess of the EPS and the CS domain, to ensure that a CSFB callsucceeds.

In the network handover methods provided in the embodiment (3) and theembodiment (4), at a stage of setting up the IMS voice session, the 5Gterminal device can be handed over to the EPS network, then the UE isenabled to fall back to a 2G/3G network by using a CSFB technology inthe EPS network, and finally a voice call is initiated in a CS domain ofthe 2G/3G network. In this application, by using an EPS network as aspringboard, and reusing a conventional 2G/3G network architecture, amethod for performing a voice service in a scenario in which an IMSvoice service may be performed in neither a 5G network nor the EPSnetwork is provided for a terminal device located in the 5G network.

In Embodiment (1) to Embodiment (4), the 500 response message may befurther a 380 response message or a 503 response message, and the RARmessage may be further an abort-session-request (ASR) message. In thiscase, the RAA message may be an abort-session-answer (ASA) message.

An embodiment of this application further provides a networkregistration method. As shown in FIG. 28A, the method includes thefollowing steps.

2801A. A first mobility management network element receives aregistration request from a terminal device, where the registrationrequest is for requesting to register the terminal device with a firstnetwork.

The first mobility management network element may be an AMF, and thefirst network may be a 5G network.

2802A. The first mobility management network element sends firstindication information to the terminal device if a third condition issatisfied, where the first indication information is for indicating thata voice service of a PS domain is supported.

The third condition may include that the first network supports movementof the terminal device to a second network. That the first networksupports movement of the terminal device to a second network may meanthat the first network supports movement of the terminal device to thesecond network in a handover or redirection manner or another manner.

Optionally, that the first network supports movement of the terminaldevice to a second network may include that a first access networkdevice in the first network supports, when receiving first sessionmanagement information from a session management network element, accessof the terminal device to the second network, where the first sessionmanagement information is for requesting to set up a user plane tunnelfor a voice service of the terminal device. The first access networkdevice may be an NG RAN, and the session management network element maybe an SMF+PGW-C function network element.

For example, the first indication information may be carried in aregistration accept message, the voice service may be an IMS voiceservice, and the terminal device may be specifically a terminal deviceinitiating a calling or called voice service.

Specifically, the first mobility management network element maydetermine, in any one of the following manners or another manner,whether the third condition is satisfied:

Manner (1): In a process in which the terminal device is registered withthe first network, the first mobility management network elementdetermines, by learning whether an access network device and/or a corenetwork element device in which the current terminal device is locatedhas a capability of moving the terminal device to the second network,whether the third condition is satisfied.

Manner (2): The first mobility management network element determines, byusing a configuration condition of the first mobility management networkelement, whether the third condition is satisfied. Whether an accessnetwork device and/or a core network element device in which the currentterminal device is located can move the terminal device to the secondnetwork is configured in the configuration condition.

Optionally, the first network and the second network have a same corenetwork and different access types; or the first network and the secondnetwork have different core networks and different access types.

When the first network and the second network have different corenetworks and different access types, the second network may be an EPSnetwork. When the first network and the second network have a same corenetwork and different access types, the first network and the secondnetwork may each be a 5G network, and have a difference in that thefirst network and the second network have different access types.

Specifically, the first access network device may enable the terminaldevice to access the second network by sending, to the first mobilitymanagement network element, a handover request for handing over theterminal device from the first network to the second network. It shouldbe noted that, when the terminal device initiates a PS domain voiceservice, the first access network device enables the terminal device toaccess the second network, and the terminal device may be a terminaldevice initiating a calling PS domain voice service or may be a terminaldevice initiating a called PS domain voice service.

Optionally, the third condition may further include that a handoverinteroperation is supported between the first network and the secondnetwork. In this case, the first network supports movement of theterminal device to the second network by performing handover.

Optionally, that a handover interoperation is supported between thefirst network and the second network includes that a handoverinteroperation interface exists between the first network and the secondnetwork. For example, the handover interoperation interface may bespecifically an N26 interface.

Optionally, the second network may support the PS domain voice serviceor may not support the PS domain voice service. If it is determined thatthe second network supports the PS domain voice service, the thirdcondition may further include that the second network can support the PSdomain voice service.

Optionally, before step 2803A, the method may further include:determining, by the first mobility management network element, that theterminal device is a voice service centric terminal device. Because adata service centric terminal device is unrelated to the voice service,the optional method may distinguish terminal types. The data servicecentric terminal device does not need to perform this set of solution,that is, does not need to determine whether the third condition issatisfied, and does not need to send the first indication informationeither.

Optionally, the method may further include: generating, by the terminaldevice, a registration request, where the registration request carriesindication information indicating that the terminal device is a voiceservice centric terminal device; and sending, by the terminal device,the registration request to the first mobility management networkelement. In this case, the determining, by the first mobility managementnetwork element, that the terminal device is a voice service centricterminal device includes: determining, by the first mobility managementnetwork element based on the registration request, that the terminaldevice is a voice service centric terminal device.

In this case, the terminal device carries, by using the registrationrequest, the indication information indicating that the terminal deviceis a voice service centric terminal device. Therefore, signaling doesnot need to be independently sent to inform the first mobilitymanagement network element of the indication information, and signalingoverheads can be saved.

Optionally, before step 2803A, the method may further include:determining, by the first mobility management network element, that theterminal device supports movement from the first network to the secondnetwork in a voice service initiation process and continuity of thevoice service initiation process by using the second network. Becausethe foregoing method may be implemented when the terminal devicesupports movement from the first network to the second network in avoice service initiation process and continuity of the voice serviceinitiation process by using the second network, the information may bedetermined in advance, to avoid impact on a terminal device that cannotsupport movement from the first network to the second network in a voiceservice initiation process and continuity of the voice serviceinitiation process by using the second network.

Optionally, the registration request further carries indicationinformation indicating that the terminal device supports movement fromthe first network to a second network in a voice service initiationprocess and continuity of the voice service initiation process by usingthe second network, and the determining, by the first mobilitymanagement network element, that the terminal device supports movementfrom the first network to the second network in a voice serviceinitiation process and continuity of the voice service initiationprocess by using the second network includes: determining, by the firstmobility management network element based on the registration request,that the terminal device supports movement from the first network to thesecond network in a voice service initiation process and continuity ofthe voice service initiation process by using the second network.

In this case, the terminal device carries, by using the registrationrequest, the indication information indicating that the terminal devicesupports movement from the first network to the second network in thevoice service initiation process and continuity of the voice serviceinitiation process by using the second network. Therefore, signalingdoes not need to be independently sent to inform the first mobilitymanagement network element of the indication information, and signalingoverheads can be saved.

It should be noted that, the method provided in this embodiment may beperformed before any method described before FIG. 28. Additionally, thefirst mobility management network element may send the first indicationinformation to the terminal device under another condition other thanthe third condition.

In the method provided in this embodiment, when the terminal device isregistered with the first network, if the first network has a capabilityof enabling, in a handover or another manner, the terminal device tofall back to a network that can provide the voice service when theterminal device initiates the voice service, the first mobilitymanagement entity indicates that the voice service in the PS domain ofthe terminal device is supported regardless of whether the first networksupports the voice service of the terminal device. The terminal deviceis registered with and camps on the first network and does not reselectanother network, thereby saving network resources, and ensuring that theterminal device normally performs a PS domain voice service registrationprocess and initiates a PS domain voice call process by using the firstnetwork.

An embodiment of this application further provides a networkregistration method. As shown in FIG. 28B, the method is used tospecifically describe the method shown in FIG. 28A. The method includesthe following steps.

2801B. UE sends a registration request to an AMF.

Specifically, the registration request is for requesting to register theUE with a first network, the UE may send the registration request to anNG RAN, and the NG RAN forwards the registration request to the AMF.

The registration request may carry indication information indicatingthat the UE is voice service centric UE, and indication informationindicating that the UE supports movement from the first network to asecond network in a voice service initiation process and continuity ofthe voice service initiation process by using the second network.

2802B. The AMF receives the registration request from the UE.

2803B. The AMF determines, based on the registration request, that theUE supports movement from a first network to a second network in a voiceservice initiation process and continuity of the voice serviceinitiation process by using the second network.

2804B. The AMF determines, based on the registration request, that theUE is voice service centric UE.

Step 2803B and step 2804B are not performed in order.

2805B. The AMF determines whether a third condition is satisfied.

If yes, step 2806B is performed. The third condition includes that thefirst network supports movement of the UE to a second network, where thesecond network can support a voice service of a PS domain. FIG. 28B isdrawn by using an example in which the AMF determines that the thirdcondition is satisfied.

2806B. The AMF sends first indication information to the UE, where thefirst indication information is for indicating that a voice service of aPS domain is supported.

The first indication information may be carried in a registration acceptmessage.

After step 2806B, the UE continues to perform a subsequent process.

The foregoing describes the solutions provided in the embodiments ofthis application mainly from the perspective of interaction betweennetwork elements. It may be understood that, to implement the foregoingfunctions, the foregoing network devices include corresponding hardwarestructures and/or software modules for performing the functions. Aperson skilled in the art should be easily aware that, in combinationwith the units and algorithm steps of the examples described in theembodiments disclosed in this specification, this application can beimplemented by hardware or a combination of hardware and computersoftware. Whether a function is performed by hardware or by computersoftware driving hardware depends on particular applications and designconstraint conditions of the technical solutions. A person skilled inthe art may use different methods to implement the described functionsfor each particular application, but it should not be considered thatthe implementation goes beyond the scope of this application.

In the embodiments of this application, the network devices may bedivided with respect to function modules based on the foregoing methodexamples. For example, function modules may be divided corresponding tofunctions, or two or more than two functions may be integrated in aprocessing module. The integrated module may be implemented in a form ofhardware, or may be implemented in a form of a software function module.It should be noted that module division in the embodiments of thisapplication is an example and is merely logical function division.During actual implementation, there may be another division manner.

For example, an example of a function module division manner of anetwork device provided in an embodiment of this application is providedbelow.

An embodiment of this application further provides a network device 290.Referring to FIG. 29, the network device 290 includes a processing unit2901 and a transceiver unit 2902.

The network device may be a first access network device, a sessionmanagement network element, a second mobility management networkelement, a second access network device, an IMS network element, or afirst mobility management network element.

Specifically, when FIG. 29 is a schematic diagram of the first accessnetwork device, the processing unit 2901 is configured to receive firstsession management information from a session management network elementby using the transceiver unit 2902, where the first session managementinformation is for requesting to set up a user plane tunnel for a voiceservice of a terminal device; and send, by using the transceiver unit2902, a handover request to a first mobility management network elementbased on the first session management information and a first condition,where the handover request is for handing over the terminal device froma first network to a second network, and the first condition includesthat the voice service cannot be performed in the first network.

Optionally, the first condition further includes at least one of thatthe second network supports the voice service, that signal quality ofthe second network satisfies a requirement of the voice service, and anetwork device of the first network and the terminal device supportshandover between the first network and the second network.

Optionally, the processing unit 2901 is further configured to sendsecond session management information to the session management networkelement by using the transceiver unit 2902, where the second sessionmanagement information includes information indicating that setup of theuser plane tunnel of the voice service is accepted.

Optionally, the processing unit 2901 is further configured to determine,based on the first session management information and the firstcondition, not to allocate a radio resource to the user plane tunnel ofthe voice service.

Optionally, the processing unit 2901 is further configured to send thirdsession management information to the session management network elementby using the transceiver unit 2902, where the third session managementinformation includes information indicating that setup of the user planetunnel of the voice service is rejected.

Optionally, the information indicating that setup of the user planetunnel of the voice service is rejected includes reason informationindicating rejection.

Optionally, the reason information is that the terminal device isperforming a handover process.

Optionally, the first condition further includes that the voice servicecannot be performed in the second network, and the processing unit 2901is further configured to send fourth session management information tothe session management network element by using the transceiver unit2902, where the fourth session management information includes theinformation indicating that setup of the user plane tunnel of the voiceservice is rejected, and the information indicating that setup of theuser plane tunnel of the voice service is rejected includes that a radioresource is insufficient.

When the terminal device in the first network performs a voice service,the network device 290 may hand over the terminal device to the secondnetwork, so that the terminal device may perform the voice service inthe second network, to resolve a problem that the terminal device in thefirst network cannot perform the voice service.

When FIG. 29 is a schematic diagram of the session management networkelement, the processing unit 2901 is configured to receive, by using thetransceiver unit 2902, third session management information sent from afirst access network device, where the third session managementinformation includes reason information indicating that setup of a userplane tunnel of a voice service of a terminal device is rejected;determine that the terminal device has handed over from a first networkto a second network; and set up the user plane tunnel for the voiceservice in the second network based on the reason information.

Optionally, the reason information is that the terminal device isperforming a handover process.

Optionally, the processing unit 2901 is specifically configured to setup the user plane tunnel for the voice service in the second networkafter a first preset period based on the reason information and a secondcondition, where the second condition includes that the voice servicecannot be performed in the second network.

Optionally, the processing unit 2901 is specifically configured to send,by using the transceiver unit 2902, a tunnel setup request to a servinggateway of the second network after the first preset period based on thereason information and the second condition, where the tunnel setuprequest is for requesting to set up the user plane tunnel of the voiceservice.

Optionally, the processing unit 2901 is specifically configured to send,by using the transceiver unit 2902, a tunnel setup request to a servinggateway of the second network based on the reason information, where thetunnel setup request is for requesting to set up the user plane tunnelof the voice service; receive, by using the transceiver unit 2902, atunnel setup response sent from the serving gateway, where the tunnelsetup response includes information indicating that setup of the userplane tunnel of the voice service is rejected; and send, by using thetransceiver unit 2902, a first message to a policy function controlnetwork element after the first preset period based on the secondcondition, where the first message is for indicating that setup of theuser plane tunnel of the voice service fails.

When the terminal device in the first network is handed over from thefirst network to the second network, the network device 290 sets up theuser plane tunnel for the voice service in the second network based onthe reason information, and if the voice service may be performed in thesecond network, the user plane tunnel may be set up successfully, sothat the terminal device may perform the voice service in the secondnetwork, to resolve a problem that the terminal device in the firstnetwork cannot perform the voice service.

When FIG. 29 is a schematic diagram of the second mobility managementnetwork element, the network device 290 may perform the following action1, action 2, action 3, or action 4.

Action 1. The processing unit 2901 is configured to receive, by usingthe transceiver unit 2902, a tunnel setup request sent by a servinggateway, where the tunnel setup request is for requesting to set up auser plane tunnel of a voice service for a terminal device; and send, byusing the transceiver unit 2902, a tunnel setup response to the servinggateway based on a second condition, where the tunnel setup responseincludes information indicating that setup of the user plane tunnel ofthe voice service of the terminal device is rejected, and the secondcondition includes that the voice service cannot be performed in asecond network.

Optionally, the second condition further includes that the terminaldevice is a terminal device handed over from a first network to thesecond network.

Optionally, the processing unit 2901 is further configured to determinethat the terminal device completes joint registration.

The processing unit 2901 is specifically configured to determine that ajoint registration complete message from the terminal device isreceived.

When the voice service cannot be performed in the second network, thenetwork device 290 sends the tunnel setup response to the servinggateway, so that the serving gateway transfers, to an IMS network, theinformation indicating that setup of the user plane tunnel is rejected,so that the IMS network may trigger a CSFB service of the terminaldevice, and then the terminal device performs a CS voice service.

Action 2. The processing unit 2901 is configured to send, by using thetransceiver unit 2902, a radio tunnel setup request to a second accessnetwork device, where the radio tunnel setup request is for requestingto set up a user plane tunnel for a voice service of a terminal device;receive, by using the transceiver unit 2902, a radio tunnel setupresponse sent by the second access network device, where the radiotunnel setup response includes information indicating that setup of theuser plane tunnel of the voice service of the terminal device isrejected; and send, by using the transceiver unit 2902, a tunnel setupresponse to a serving gateway based on a second condition, where thetunnel setup response includes the information indicating that setup ofthe user plane tunnel of the voice service of the terminal device isrejected, and the second condition includes that the voice servicecannot be performed in a second network.

Optionally, the second condition further includes that the terminaldevice is a terminal device handed over from a first network to thesecond network.

Optionally, the processing unit 2901 is specifically configured todetermine that the second condition is satisfied and the terminal devicecompletes joint registration; and send the tunnel setup response to theserving gateway by using the transceiver unit 2902.

Optionally, the processing unit 2901 is specifically configured todetermine that a registration complete message from the terminal deviceis received.

The network device 290 sends, to the serving gateway, the informationindicating that setup of the user plane tunnel is rejected, so that theserving gateway transfers, to an IMS network, the information indicatingthat setup of the user plane tunnel is rejected, the IMS networktriggers a CSFB service of the terminal device, and then the terminaldevice performs a CS voice service.

Action 3. The processing unit 2901 is configured to receive, by usingthe transceiver unit 2902, a tunnel setup request sent by a servinggateway, where the tunnel setup request is for requesting to set up auser plane tunnel of a voice service for user equipment, namely, aterminal device; and if a second condition is satisfied, determine thatthe terminal device completes joint registration, where the secondcondition includes that the voice service cannot be performed in asecond network; send, by using the transceiver unit 2902, a radio tunnelsetup request to a second access network device, where the radio tunnelsetup request is for requesting to set up the user plane tunnel for thevoice service of the terminal device; receive, by using the transceiverunit 2902, a radio tunnel setup response sent by the second accessnetwork device, where the radio tunnel setup response includesinformation indicating that setup of the user plane tunnel of the voiceservice of the terminal device is rejected; and send, by using thetransceiver unit 2902, a tunnel setup response to the serving gatewaybased on the radio tunnel setup response, where the tunnel setupresponse includes the information indicating that setup of the userplane tunnel of the voice service of the terminal device is rejected.

Optionally, the second condition further includes that the terminaldevice is a terminal device handed over from a first network to thesecond network.

Optionally, the processing unit 2901 is specifically configured todetermine that a registration complete message sent by the terminaldevice is received.

After determining that the terminal device completes joint registration,the network device 290 sends, to the serving gateway, the informationindicating that setup of the user plane tunnel is rejected, so that theserving gateway transfers, to an IMS network, the information indicatingthat setup of the user plane tunnel is rejected, the IMS networktriggers a CSFB service of the terminal device, and then the terminaldevice performs a CS voice service, to further ensure that the terminaldevice successfully initiates a CSFB call.

Action 4. The processing unit 2901 is configured to receive, by usingthe transceiver unit 2902, a fourth message sent by a first mobilitymanagement network element, where the fourth message is for determiningthat a terminal device completes relocation; and send, by using thetransceiver unit 2902, a tunnel modification request to a servinggateway based on a second condition, where the tunnel modificationrequest is for modifying signaling plane tunnel information of a voiceservice of the terminal device, and the second condition includes thatthe voice service cannot be performed in a second network.

Optionally, the processing unit 2901 is specifically configured todetermine that the second condition is satisfied and the UE completesjoint registration; and send the tunnel modification request to theserving gateway by using the transceiver unit 2902.

When the voice service cannot be performed in the second network, thenetwork device 290 sends the tunnel modification request to the servinggateway, the serving gateway forwards the tunnel modification request toa session management network element, and because only after receivingthe tunnel modification request, the session management network elementsends, to the terminal device, a message (that is, the foregoing secondmessage) for triggering the terminal device to initiate a CSFB call, thenetwork device 290 may trigger the terminal device to initiate the CSFBcall when the voice service cannot be performed in the second network,so that the terminal device performs a CS voice service.

When FIG. 29 is a schematic diagram of the second access network device,the processing unit 2901 is configured to receive, by using thetransceiver unit 2902, a radio tunnel setup request sent by a secondmobility management network element, where the radio tunnel setuprequest is for requesting the second access network device to set up auser plane tunnel of a voice service for a terminal device; and send, byusing the transceiver unit 2902, a radio tunnel setup response to thesecond mobility management network element based on a second condition,where the radio tunnel setup response includes information indicatingthat setup of the user plane tunnel of the voice service is rejected,and the second condition includes that the voice service cannot beperformed in a second network.

Optionally, the second condition further includes that the terminaldevice is a terminal device handed over from a first network to thesecond network.

When the voice service cannot be performed in the second network, thenetwork device 290 rejects allocating a radio resource to the user planetunnel, adds, to the radio tunnel setup response, the informationindicating that setup of the user plane tunnel of the voice service isrejected, and sends the radio tunnel setup response to the secondmobility management network element, and the second mobility managementnetwork element may transfer, to an IMS network, the informationindicating that setup of the user plane tunnel is rejected, so that theIMS network triggers a CSFB service of the terminal device, and then theterminal device performs a CS voice service.

When FIG. 29 is a schematic diagram of the IMS network element, theprocessing unit 2901 is configured to receive, by using the transceiverunit 2902, a request message for setting up a voice service for aterminal device in a packet switched PS domain; determine that the voiceservice cannot be set up in a PS domain of a first network or a secondnetwork; determine that the terminal device completes jointregistration; and trigger a circuit switched fallback CSFB service ofthe terminal device.

Optionally, the processing unit 2901 is further configured to determinethat the terminal device is registered with the first network.

Optionally, the processing unit 2901 is specifically configured toobtain voice service setup failure information that is from the PSdomain of the first network; or obtain voice service setup failureinformation that is from the PS domain of the second network.

Optionally, the processing unit 2901 is specifically configured to:after a second preset period since the IMS network element determinesthat the voice service cannot be set up in the PS domain of the firstnetwork or the second network, determine that the terminal devicecompletes joint registration; or determine, based on registrationlocation information of the terminal device obtained from a policycontrol network element or a home subscriber server, that the terminaldevice completes joint registration.

Optionally, the terminal device is a calling terminal device of thevoice service, and the processing unit 2901 is specifically configuredto send, by using the transceiver unit 2902, a second message to thecalling terminal device, where the second message is for triggering thecalling terminal device to initiate a CSFB calling service.

Optionally, the terminal device is a called terminal device of the voiceservice, and the processing unit 2901 is specifically configured tosend, by using the transceiver unit 2902, a third message to aterminating access domain selection device, where the third message isfor indicating that setup of a called voice service in the PS domain ofthe first network or the second network fails, so that the terminatingaccess domain selection device triggers a CSFB called voice service ofthe called terminal device.

When determining that a voice service cannot be set up in the PS domainof the first network or the second network, and the terminal devicecompletes joint registration, the network device 290 triggers the CSFBservice of the terminal device. Because that the terminal devicecompletes joint registration is a necessary condition on which theterminal device initiates a CSFB call, it may be ensured that theterminal device successfully initiates the CSFB call, so that theterminal device performs a CS voice service.

When FIG. 29 is a schematic diagram of the first mobility managementnetwork element, the processing unit 2901 is configured to receive aregistration request from a terminal device by using the transceiverunit 2902, where the registration request is for requesting to registerthe terminal device with a first network; the processing unit 2901 isfurther configured to determine that a third condition is satisfied,where the third condition includes that the first network supportsmovement of the terminal device to a second network, where the secondnetwork can support a voice service of a PS domain; and the processingunit 2901 is further configured to send first indication information tothe terminal device by using the transceiver unit 2902, where the firstindication information is for indicating that a voice service of a PSdomain is supported.

Optionally, the first network and the second network have a same corenetwork and different access types; or the first network and the secondnetwork have different core networks and different access types.

Optionally, that the first network supports movement of the terminaldevice to a second network includes that a first access network devicein the first network supports, when receiving first session managementinformation from a session management network element, access of theterminal device to the second network, where the first sessionmanagement information is for requesting to set up a user plane tunnelfor a voice service of the terminal device.

Optionally, the third condition further includes that a handoverinteroperation is supported between the first network and the secondnetwork.

Optionally, that a handover interoperation is supported between thefirst network and the second network includes that a handoverinteroperation interface exists between the first network and the secondnetwork.

Optionally, the processing unit 2901 is further configured to determinethat the terminal device is a voice service centric terminal device.

Optionally, the registration request carries indication informationindicating that the terminal device is a voice service centric terminaldevice, and the processing unit 2901 is specifically configured todetermine, based on the registration request, that the terminal deviceis a voice service centric terminal device.

Optionally, the processing unit 2901 is further configured to determinethat the terminal device supports movement from the first network to thesecond network in a voice service initiation process and continuity ofthe voice service initiation process by using the second network.

Optionally, the registration request further carries indicationinformation indicating that the terminal device supports movement fromthe first network to a second network in a voice service initiationprocess and continuity of the voice service initiation process by usingthe second network, and the processing unit 2901 is specificallyconfigured to determine, based on the registration request, that theterminal device supports movement from the first network to the secondnetwork in a voice service initiation process and continuity of thevoice service initiation process by using the second network.

Regardless of whether the first network supports the voice service ofthe terminal device, provided that the first network has a capability ofenabling, in a handover or another manner, the terminal device to fallback to a network that can provide the voice service when the terminaldevice initiates the voice service, when the terminal device isregistered with the first network, the first mobility management networkelement indicates that the voice service in the PS domain of theterminal device is supported, thereby ensuring that the terminal deviceis registered with and camps on the first network and does not reselectanother network, thereby saving network resources, and ensuring that theterminal device normally performs a PS domain voice service registrationprocess and initiates a PS domain voice call process by using the firstnetwork.

An embodiment of this application further provides a terminal device290A. Referring to FIG. 29A, the terminal device 290A includes aprocessing unit 2901A and a transceiver unit 2902A. The processing unit2901A is configured to generate a registration request, where theregistration request is for requesting to register the terminal devicewith a first network, and the registration request carries indicationinformation indicating that the terminal device is a voice servicecentric terminal device; and the processing unit 2901A is furtherconfigured to send the registration request to a first mobilitymanagement network element by using the transceiver unit 2902A.

Optionally, the registration request further includes indicationinformation indicating that the terminal device supports movement fromthe first network to a second network in a voice service initiationprocess and continuity of the voice service initiation process by usingthe second network.

Optionally, the first network and the second network have a same corenetwork and different access types; or the first network and the secondnetwork have different core networks and different access types.

When the first network has a capability of enabling, in a handover oranother manner, the terminal device to fall back to a network that canprovide the voice service when the terminal device initiates the voiceservice, the terminal device is registered with and camps on, based onthe received first indication information sent by the first mobilitymanagement network element, the first network and does not reselectanother network, thereby saving network resources, and ensuring that theterminal device normally performs a PS domain voice service registrationprocess and initiates a PS domain voice call process by using the firstnetwork.

An embodiment of this application further provides a network device. Thenetwork device includes: a memory, a processor, and a communicationsinterface. The memory is configured to store a computer executableinstruction, and the processor executes the computer executableinstruction stored in the memory, and enables, by using thecommunications interface, the network device to implement a methodcorresponding to the instruction. For a schematic structural diagram ofthe network device, refer to FIG. 6.

The network device may be a first access network device, a sessionmanagement network element, a second mobility management networkelement, a second access network device, an IMS network element, or afirst mobility management network element. Specifically, when FIG. 6 andFIG. 29 are schematic diagrams of the first access network device, theprocessing unit 2901 may be the processor 601, and the transceiver unit2902 may be the communications interface 604. When FIG. 6 and FIG. 29are schematic diagrams of the session management network element, theprocessing unit 2901 may be the processor 601, and the transceiver unit2902 may be the communications interface 604. When FIG. 6 and FIG. 29are schematic diagrams of the second mobility management networkelement, the processing unit 2901 may be the processor 601, and thetransceiver unit 2902 may be the communications interface 604. When FIG.6 and FIG. 29 are schematic diagrams of the second access networkdevice, the processing unit 2901 may be the processor 601, and thetransceiver unit 2902 may be the communications interface 604. When FIG.6 and FIG. 29 are schematic diagrams of the IMS network element, theprocessing unit 2901 may be the processor 601, and the transceiver unit2902 may be the communications interface 604. When FIG. 6 and FIG. 29are schematic diagrams of the first mobility management network element,the processing unit 2901 may be the processor 601, and the transceiverunit 2902 may be the communications interface 604.

An embodiment of this application further provides a terminal device.The terminal device includes: a memory, a processor, and acommunications interface. The memory is configured to store a computerexecutable instruction, and the processor executes the computerexecutable instruction stored in the memory, and enables, by using thecommunications interface, the terminal device to implement a methodcorresponding to the instruction. For a schematic structural diagram ofthe terminal device, refer to FIG. 6A. The processing unit 2901A may bethe processor 601A, and the transceiver unit 2902A may be thecommunications interface 604A.

An embodiment of this application further provides a computer readablestorage medium. The computer readable storage medium includes aninstruction, and when run on a computer, the instruction enables thecomputer to perform the foregoing method.

An embodiment of this application further provides a computer programproduct including an instruction. When run on a computer, theinstruction enables the computer to perform the foregoing method.

All or some of the foregoing embodiments may be implemented by usingsoftware, hardware, firmware, or any combination thereof. When asoftware program is used to implement the embodiments, the embodimentsmay be implemented completely or partially in a form of a computerprogram product. The computer program product includes one or morecomputer instructions. When the computer program instructions are loadedand executed on the computer, the procedure or functions according tothe embodiments of this application are all or partially generated. Thecomputer may be a general-purpose computer, a dedicated computer, acomputer network, or other programmable apparatuses. The computerinstructions may be stored in a computer-readable storage medium or maybe transmitted from a computer-readable storage medium to anothercomputer-readable storage medium. For example, the computer instructionsmay be transmitted from a website, computer, server, or data center toanother website, computer, server, or data center in a wired (forexample, a coaxial cable, an optical fiber, or a digital subscriber line(DSL)) or wireless (for example, infrared, radio, and microwave, or thelike) manner. The computer-readable storage medium may be any usablemedium accessible by a computer, or a data storage device, such as aserver or a data center, integrating one or more usable media. Theusable medium may be a magnetic medium (for example, a soft disk, a harddisk, or a magnetic tape), an optical medium (for example, a DVD), asemiconductor medium (for example, a solid-state drive (SSD)), or thelike.

Although this application is described with reference to theembodiments, in a process of implementing this application that claimsprotection, a person skilled in the art may understand and implementanother variation of the disclosed embodiments by viewing theaccompanying drawings, disclosed content, and the accompanying claims.In the claims, “comprising” does not exclude another component oranother step, and “a” or “one” does not exclude a case of multiple. Asingle processor or another unit may implement several functionsenumerated in the claims. Some measures are recorded in dependent claimsthat are different from each other, but this does not mean that thesemeasures cannot be combined to produce a better effect.

Although this application is described with reference to specificfeatures and the embodiments thereof, obviously, various modificationsand combinations may be made to them without departing from the spiritand scope of this application. Correspondingly, the specification andaccompanying drawings are merely example description of this applicationdefined by the accompanying claims, and is considered as any of or allmodifications, variations, combinations or equivalents that cover thescope of this application. Obviously, a person skilled in the art canmake various modifications and variations to this application withoutdeparting from the spirit and scope of this application. Thisapplication is intended to cover these modifications and variations ofthis application provided that they fall within the scope of protectiondefined by the following claims and their equivalent technologies.

What is claimed is:
 1. A network registration method, comprising:receiving, by a first mobility management network element, aregistration request from a terminal device, wherein the registrationrequest is for requesting to register the terminal device with a firstnetwork; and sending, by the first mobility management network element,indication information to the terminal device if a condition issatisfied, wherein the indication information indicating that a voiceservice of a packet switched (PS) domain is supported, and the conditioncomprises that the first network supports movement of the terminaldevice to a second network.
 2. The network registration method accordingto claim 1, wherein that the first network supports movement of theterminal device to a second network comprises that the first networksupports movement of the terminal device to the second network in ahandover or redirection manner.
 3. The network registration methodaccording to claim 1, wherein the first network and the second networkhave a same core network and different access types.
 4. The networkregistration method according to claim 3, wherein that the first networkand the second network have a same core network and different accesstypes comprises that the core network of the first network and thesecond network is a fifth-generation (5G) core network, an access typeof the first network is a next generation radio access network (NG RAN),and an access type of the second network is an evolved universalterrestrial radio access network (E-UTRAN).
 5. The network registrationmethod according to claim 1, wherein the first network and the secondnetwork have different core networks and different access types.
 6. Thenetwork registration method according to claim 5, wherein that the firstnetwork and the second network have different core networks anddifferent access types comprises that a core network of the firstnetwork is a 5G core network, a core network of the second network is afourth-generation (4G) core network, an access type of the first networkis an NG RAN, and an access type of the second network is an E-UTRAN. 7.The network registration method according to claim 1, wherein that thefirst network supports movement of the terminal device to a secondnetwork comprises that a first access network device in the firstnetwork supports, when receiving session management information from asession management network element, access of the terminal device to thesecond network, wherein the session management information is forrequesting to set up a user plane tunnel for a voice service of theterminal device.
 8. The network registration method according to claim1, wherein the condition further comprises that the second networksupport the voice service of the PS domain.
 9. The network registrationmethod according to claim 1, wherein the first network supports movementof the terminal device to the second network comprises: the firstnetwork supports movement of the terminal device to the second networkthrough a handover interoperation interface exists between the firstnetwork and the second network.
 10. The network registration methodaccording to claim 1, wherein before the sending indication informationto the terminal device, the network registration method furthercomprises: determining, by the first mobility management networkelement, that the terminal device is a voice service centric terminaldevice.
 11. The network registration method according to claim 10,wherein the registration request carries indication informationindicating that the terminal device is a voice service centric terminaldevice, and the determining that the terminal device is a voice servicecentric terminal device comprises: determining, by the first mobilitymanagement network element based on the registration request, that theterminal device is a voice service centric terminal device.
 12. Anetwork device, comprising: a communications interface, configured toreceive a registration request from a terminal device, wherein theregistration request is for requesting to register the terminal devicewith a first network; and at least one processor, configured todetermine that a condition is satisfied in response to the registrationrequest; wherein the communications interface is further configured tosend indication information to the terminal device when the condition issatisfied, the indication information indicating that a voice service ofa packet switched (PS) domain is supported, and the condition comprisesthat the first network supports movement of the terminal device to asecond network.
 13. The network device according to claim 12, whereinthat the first network supports movement of the terminal device to asecond network comprises that the first network supports movement of theterminal device to the second network in a handover or redirectionmanner.
 14. The network device according to claim 12, wherein a corenetwork of the first network and the second network is afifth-generation (5G) core network, an access type of the first networkis a next generation radio access network (NG RAN), and an access typeof the second network is an evolved universal terrestrial radio accessnetwork (E-UTRAN).
 15. The network device according to claim 12, whereina core network of the first network is a 5G core network, a core networkof the second network is a fourth-generation (4G) core network, anaccess type of the first network is an NG RAN, and an access type of thesecond network is an E-UTRAN.
 16. The network device according to claim12, wherein that the first network supports movement of the terminaldevice to a second network comprises that a first access network devicein the first network supports, when receiving session managementinformation from a session management network element, access of theterminal device to the second network, wherein the session managementinformation is for requesting to set up a user plane tunnel for a voiceservice of the terminal device.
 17. The network device according toclaim 12, wherein the condition further comprises that the secondnetwork support the voice service of the PS domain.
 18. The networkdevice according to claim 12, wherein the first network supportsmovement of the terminal device to the second network comprises: thefirst network supports movement of the terminal device to the secondnetwork through a handover interoperation interface exists between thefirst network and the second network.
 19. The network device accordingto claim 12, wherein the at least one processor is further configuredto: determine that the terminal device is a voice service centricterminal device.
 20. A non-transitory computer-readable storage mediumstoring a program to be executed by at least one processor of a device,the program including instructions to cause the device to carry out thefollowing operations: receiving a registration request from a terminaldevice, wherein the registration request is for requesting to registerthe terminal device with a first network; and sending indicationinformation to the terminal device if a condition is satisfied, whereinthe indication information indicating that a voice service of a packetswitched (PS) domain is supported, and the condition comprises that thefirst network supports movement of the terminal device to a secondnetwork.